Redmine

First thing first:

Environment:
  Redmine version                          2.3.1.stable
  Ruby version                             1.8.7 (x86_64-linux)
  Rails version                            3.2.13
  Environment                              production
  Database adapter                         MySQL
Redmine plugins:
  ekanban                                  0.0.1
Server:
  Ubuntu Linux 12.04.3 LTS

I'm trying to upgrade from our redmine version to newest 2.5.2, I'm following http://www.redmine.org/projects/redmine/wiki/RedmineUpgrade
Everything is fine until:

rake db:migrate RAILS_ENV=production

I'm getting following error:

** Invoke db:migrate (first_time)
** Invoke environment (first_time)
** Execute environment
rake aborted!
uninitialized constant MessageObserver
/var/lib/gems/1.8/gems/activesupport-3.2.19/lib/active_support/inflector/methods.rb:219:in `constantize'
/var/lib/gems/1.8/gems/activesupport-3.2.19/lib/active_support/inflector/methods.rb:218:in `each'
/var/lib/gems/1.8/gems/activesupport-3.2.19/lib/active_support/inflector/methods.rb:218:in `constantize'
/var/lib/gems/1.8/gems/activesupport-3.2.19/lib/active_support/core_ext/string/inflections.rb:54:in `constantize'
/var/lib/gems/1.8/gems/activemodel-3.2.19/lib/active_model/observing.rb:86:in `instantiate_observer'
/var/lib/gems/1.8/gems/activemodel-3.2.19/lib/active_model/observing.rb:59:in `instantiate_observers'
/var/lib/gems/1.8/gems/activemodel-3.2.19/lib/active_model/observing.rb:59:in `each'
/var/lib/gems/1.8/gems/activemodel-3.2.19/lib/active_model/observing.rb:59:in `instantiate_observers'
/var/lib/gems/1.8/gems/activerecord-3.2.19/lib/active_record/railtie.rb:123
/var/lib/gems/1.8/gems/activesupport-3.2.19/lib/active_support/lazy_load_hooks.rb:36:in `instance_eval'
/var/lib/gems/1.8/gems/activesupport-3.2.19/lib/active_support/lazy_load_hooks.rb:36:in `execute_hook'
/var/lib/gems/1.8/gems/activesupport-3.2.19/lib/active_support/lazy_load_hooks.rb:26:in `on_load'
/var/lib/gems/1.8/gems/activesupport-3.2.19/lib/active_support/lazy_load_hooks.rb:25:in `each'
/var/lib/gems/1.8/gems/activesupport-3.2.19/lib/active_support/lazy_load_hooks.rb:25:in `on_load'
/var/lib/gems/1.8/gems/activerecord-3.2.19/lib/active_record/railtie.rb:122
/var/lib/gems/1.8/gems/activesupport-3.2.19/lib/active_support/lazy_load_hooks.rb:34:in `call'
/var/lib/gems/1.8/gems/activesupport-3.2.19/lib/active_support/lazy_load_hooks.rb:34:in `execute_hook'
/var/lib/gems/1.8/gems/activesupport-3.2.19/lib/active_support/lazy_load_hooks.rb:43:in `run_load_hooks'
/var/lib/gems/1.8/gems/activesupport-3.2.19/lib/active_support/lazy_load_hooks.rb:42:in `each'
/var/lib/gems/1.8/gems/activesupport-3.2.19/lib/active_support/lazy_load_hooks.rb:42:in `run_load_hooks'
/var/lib/gems/1.8/gems/railties-3.2.19/lib/rails/application/finisher.rb:59
/var/lib/gems/1.8/gems/railties-3.2.19/lib/rails/initializable.rb:30:in `instance_exec'
/var/lib/gems/1.8/gems/railties-3.2.19/lib/rails/initializable.rb:30:in `run'
/var/lib/gems/1.8/gems/railties-3.2.19/lib/rails/initializable.rb:55:in `run_initializers'
/var/lib/gems/1.8/gems/railties-3.2.19/lib/rails/initializable.rb:54:in `each'
/var/lib/gems/1.8/gems/railties-3.2.19/lib/rails/initializable.rb:54:in `run_initializers'
/var/lib/gems/1.8/gems/railties-3.2.19/lib/rails/application.rb:136:in `initialize!'
/var/lib/gems/1.8/gems/railties-3.2.19/lib/rails/railtie/configurable.rb:30:in `send'
/var/lib/gems/1.8/gems/railties-3.2.19/lib/rails/railtie/configurable.rb:30:in `method_missing'
/usr/share/redmine/config/environment.rb:14
/var/lib/gems/1.8/gems/railties-3.2.19/lib/rails/application.rb:103:in `require_environment!'
/var/lib/gems/1.8/gems/railties-3.2.19/lib/rails/application.rb:305:in `initialize_tasks'
/var/lib/gems/1.8/gems/rake-10.1.1/lib/rake/task.rb:236:in `call'
/var/lib/gems/1.8/gems/rake-10.1.1/lib/rake/task.rb:236:in `execute'
/var/lib/gems/1.8/gems/rake-10.1.1/lib/rake/task.rb:231:in `each'
/var/lib/gems/1.8/gems/rake-10.1.1/lib/rake/task.rb:231:in `execute'
/var/lib/gems/1.8/gems/rake-10.1.1/lib/rake/task.rb:175:in `invoke_with_call_chain'
/usr/lib/ruby/1.8/monitor.rb:242:in `synchronize'
/var/lib/gems/1.8/gems/rake-10.1.1/lib/rake/task.rb:168:in `invoke_with_call_chain'
/var/lib/gems/1.8/gems/rake-10.1.1/lib/rake/task.rb:197:in `invoke_prerequisites'
/var/lib/gems/1.8/gems/rake-10.1.1/lib/rake/task.rb:195:in `each'
/var/lib/gems/1.8/gems/rake-10.1.1/lib/rake/task.rb:195:in `invoke_prerequisites'
/var/lib/gems/1.8/gems/rake-10.1.1/lib/rake/task.rb:174:in `invoke_with_call_chain'
/usr/lib/ruby/1.8/monitor.rb:242:in `synchronize'
/var/lib/gems/1.8/gems/rake-10.1.1/lib/rake/task.rb:168:in `invoke_with_call_chain'
/var/lib/gems/1.8/gems/rake-10.1.1/lib/rake/task.rb:161:in `invoke'
/var/lib/gems/1.8/gems/rake-10.1.1/lib/rake/application.rb:149:in `invoke_task'
/var/lib/gems/1.8/gems/rake-10.1.1/lib/rake/application.rb:106:in `top_level'
/var/lib/gems/1.8/gems/rake-10.1.1/lib/rake/application.rb:106:in `each'
/var/lib/gems/1.8/gems/rake-10.1.1/lib/rake/application.rb:106:in `top_level'
/var/lib/gems/1.8/gems/rake-10.1.1/lib/rake/application.rb:115:in `run_with_threads'
/var/lib/gems/1.8/gems/rake-10.1.1/lib/rake/application.rb:100:in `top_level'
/var/lib/gems/1.8/gems/rake-10.1.1/lib/rake/application.rb:78:in `run'
/var/lib/gems/1.8/gems/rake-10.1.1/lib/rake/application.rb:165:in `standard_exception_handling'
/var/lib/gems/1.8/gems/rake-10.1.1/lib/rake/application.rb:75:in `run'
/var/lib/gems/1.8/gems/rake-10.1.1/bin/rake:33
/usr/local/bin/rake:19:in `load'
/usr/local/bin/rake:19
Tasks: TOP => db:migrate => environment

I've googled quite a lot about the issue, but there seeems to be no clue about what to do with this problem.
Only one making sense was http://www.rubydoc.info/github/asoltys/redmine/MessageObserver
But there is no such file as MessageObserver.rb in https://github.com/edavis10/redmine/tree/master/app/models

Bundle:

Using rake (10.1.1)
Using i18n (0.6.11)
Using multi_json (1.10.1)
Using activesupport (3.2.19)
Using builder (3.0.0)
Using activemodel (3.2.19)
Using erubis (2.7.0)
Using journey (1.0.4)
Using rack (1.4.5)
Using rack-cache (1.2)
Using rack-test (0.6.2)
Using hike (1.2.3)
Using tilt (1.4.1)
Using sprockets (2.2.2)
Using actionpack (3.2.19)
Using mime-types (1.25.1)
Using polyglot (0.3.5)
Using treetop (1.4.15)
Using mail (2.5.4)
Using actionmailer (3.2.19)
Using arel (3.0.3)
Using tzinfo (0.3.40)
Using activerecord (3.2.19)
Using activeresource (3.2.19)
Using awesome_nested_set (2.1.6)
Using coderay (1.1.0)
Using fastercsv (1.5.5)
Using rack-ssl (1.3.4)
Using json (1.8.1)
Using rdoc (3.12.2)
Using thor (0.19.1)
Using railties (3.2.19)
Using jquery-rails (2.0.3)
Using mysql (2.8.1)
Using net-ldap (0.3.1)
Using ruby-openid (2.3.0)
Using rack-openid (1.4.2)
Using bundler (1.1.4)
Using rails (3.2.19)
Using redcarpet (2.3.0)