fix rails 4.2 deprecation warning in uniqueness validator

Running mongoid/master against rails/master generates deprecation warnings on the uniqueness validator, as per:

https://github.com/rails/rails/blob/master/activemodel/lib/active_model/validator.rb#L129

I've removed the setup, and used document.class.superclass to locate the appropriate scope for the uniqueness query to be run from. Specs are all still green. What have I missed?