Added a fix to allow this gem to work with MongoDB 2.x
MongoDB 2.x returns nil instead of raising Mongo::OperationFailure.
I also started messing with adding support for Mongoid 3 but had to back
off because it will involve some refactoring.  I left in some comments
to try to help.
Merge pull request #1 from joealba/mongo2_fix

Support for MongoDB 2.x