Bundler could not find compatible versions for gem "activemodel":
  In Gemfile:
    mongoid (~> 2.2.6) ruby depends on
      activemodel (~> 3.0.0) ruby
    rails (= 3.2.13) ruby depends on
      activemodel (3.2.13)

 Mongoid cannot be used with Rails 3.2 and REE or 1.8.7