Test as collection of Mongoid 4 documents.

Current spec status:
  standalone: 89 examples, 0 failures in 0.12638 seconds.
  ActiveRecord 4: 166 examples, 0 failures in 0.20171 seconds.
  Mongoid 4: 166 examples, 0 failures in 0.21754 seconds.
Require mongodb service on Travis builds.
Merge pull request #3 from sleepingkingstudios/mongoid-4

Test as collection of Mongoid 4 documents.