Remove bson_ext gem when using Mongoid

Mongoid3 no longer uses the bson_ext gem, in preference to its own
Moped::BSON implementation.
http://mongoid.org/en/mongoid/docs/upgrading.html

One of my co-workers also found a performance problem when mongoid3 and bson_ext are used https://github.com/brianhempel/mongo_session_store/issues/18.
I think that this change will resolve that issue.