Add Gemfile.mongoid-four to test Mongoid 4.0.0 from master
Fix compatibility with Mongoid 4 in ToCallback#define_callback
Merge pull request #31 from gevans/mongoid-4-compatibility

Mongoid 4 compatibility
Add spec support for mongoid4
Add methods #relation_pull, #relation_push, and rename #mongoid4? to #mongoid_4 for consistency