Introduce rails_12factor gem

Added `rails_12factor` to prevent following warnings on heroku deployment:
```
-----> Preparing app for Rails asset pipeline
       Running: rake assets:precompile
       DEPRECATION WARNING: You have Rails 2.3-style plugins in vendor/plugins! Support for these plugins will be removed in Rails 4.0. Move them out and bundle them in your Gemfile, or fold them in to your app as lib/myplugin/* and config/initializers/myplugin.rb. See the release notes for more on this: http://weblog.rubyonrails.org/2012/1/4/rails-3-2-0-rc2-has-been-released. (called from <top (required)> at /tmp/build_035e79e5-f331-4a15-832e-1b53ff06ff12/Rakefile:7)
       DEPRECATION WARNING: You have Rails 2.3-style plugins in vendor/plugins! Support for these plugins will be removed in Rails 4.0. Move them out and bundle them in your Gemfile, or fold them in to your app as lib/myplugin/* and config/initializers/myplugin.rb. See the release notes for more on this: http://weblog.rubyonrails.org/2012/1/4/rails-3-2-0-rc2-has-been-released. (called from <top (required)> at /tmp/build_035e79e5-f331-4a15-832e-1b53ff06ff12/Rakefile:7)
       DEPRECATION WARNING: You have Rails 2.3-style plugins in vendor/plugins! Support for these plugins will be removed in Rails 4.0. Move them out and bundle them in your Gemfile, or fold them in to your app as lib/myplugin/* and config/initializers/myplugin.rb. See the release notes for more on this: http://weblog.rubyonrails.org/2012/1/4/rails-3-2-0-rc2-has-been-released. (called from <top (required)> at /tmp/build_035e79e5-f331-4a15-832e-1b53ff06ff12/Rakefile:7)
       DEPRECATION WARNING: You have Rails 2.3-style plugins in vendor/plugins! Support for these plugins will be removed in Rails 4.0. Move them out and bundle them in your Gemfile, or fold them in to your app as lib/myplugin/* and config/initializers/myplugin.rb. See the release notes for more on this: http://weblog.rubyonrails.org/2012/1/4/rails-3-2-0-rc2-has-been-released. (called from <top (required)> at /tmp/build_035e79e5-f331-4a15-832e-1b53ff06ff12/Rakefile:7)
       Asset precompilation completed (38.37s)
-----> WARNINGS:
       Injecting plugin 'rails_log_stdout'
       Injecting plugin 'rails3_serve_static_assets'
       Add 'rails_12factor' gem to your Gemfile to skip plugin injection
```