rails: get uninitialized constant ActiveSupport::Logger::SimpleFormatter everytime I try to run my specs

Question

I use both rake spec and rspec spec/ trying to run my rspec tests, but I get the following error:

/home/XXX/.rvm/gems/ruby-2.2.1/gems/railties-4.2.1/lib/rails/application/configuration.rb:49:in `initialize': uninitialized constant ActiveSupport::Logger::SimpleFormatter (NameError)
  from /home/XXX/.rvm/gems/ruby-2.2.1/gems/railties-4.2.1/lib/rails/application.rb:364:in `new'
  from /home/XXX/.rvm/gems/ruby-2.2.1/gems/railties-4.2.1/lib/rails/application.rb:364:in `config'
  from /home/XXX/.rvm/gems/ruby-2.2.1/gems/railties-4.2.1/lib/rails/railtie.rb:123:in `config'
  from /home/XXX/proj/candiru/config/application.rb:26:in `<class:Application>'
  from /home/XXX/proj/candiru/config/application.rb:11:in `<module:Candiru>'

It's weird because this file (config/application.rb) is loaded when I start the rails server and rails console, without any problems.

The "suspicious" line is: config.assets.paths << Rails.root.join("vendor","assets","bower_components"), and if I comment it out, the next similar configuration line will marked as the error line.

Do you have any guess why this error is thrown?