Relax dependency on Mongoid 3.2.x, remove Gemfile.lock.

Mongoid is moving forward with the 3.1.x line, and mongoid-locker works with it just fine. 

I also removed Gemfile.lock which locks gems at certain versions for testing, while we want the latest compatible gems most of the time (otherwise people getting the gem might run into bugs that aren't caught in testing as they aren't using mongoid-locker's Gemfile.lock).