[MODEL] Removed the "mongoid" gem from the 4.x gemfile, because of dependency resolution failures

Bundler could not find compatible versions for gem "minitest":
  In Gemfile:
    mongoid (= 4.0.0) ruby depends on
      activemodel (~> 4.0) ruby depends on
        activesupport (= 4.0.0) ruby depends on
          minitest (~> 4.2) ruby

    minitest (4.0.0)