Merge pull request #2 from deevis/removing-identity-map

Removing IdentityMap as it is no longer part of Mongoid
Update README.md

[ci skip]
Correcting dependency on Mongoid
Remove versions on document clone
Merge pull request #3 from arthurnn/versions_on_copy

Remove versions on document clone
Stick with newest mongoid.
Use proc syntax for scope in test.
Mongoid changed criteria_instance_id to __selected_fields