Add mongoid2 support and add mongoid2 to travis test matrix.
Only re-define BSON::ObjectId's <=> operator for mongoid2.
Redefine BSON::ObjectId <=> method to compare binary data.
Merge branch 'mongoid2-support' of https://github.com/joe1chen/searchkick into joe1chen-mongoid2-support
Mongoid2 updates
Merge branch 'joe1chen-mongoid2-support'