Update the mocha require command

Deprecation message indicated that this change should be made.
Add rake to the development requirements

Need this to get rake to run properly within a bundle exec call.
Update the gem to support Mongoid3

The test configuration has to be changed and the bson_ext requirement
can be removed, as described in the Mongoid upgrade document.
http://mongoid.org/en/mongoid/docs/upgrading.html