Upgrade the gem to use Mongoid3

Removed the bson_ext requirement, add a dependency on Mongoid3 or greater, and updated the test to use the Mongoid3 configuration method.
The test suite passes for me, and we'll be trying the modified gem out in our project in the next few days. I expect it will work, but I send updates if there are any problems.

Unrelated, I also needed to add rake into the development dependencies and updated the mocha require command based upon their deprecation message. There were convenient changes to have, but if you'd like the PR to be simpler I can certainly remove them from the branch.

Thanks in advance, and for releasing this gem.