Mongoid compatibility

Useful gem! In one of my Rails apps, Mongoid is used over ActiveRecord which results in some errors due to the initializer. I updated the it to only include the msgpack serializer mixin when `ActiveRecord::Base` is defined and loaded. Additionally, I added to the initializer to do the same for `Mongoid::Document`.

To run the test suite I needed to lock minitest to `~> 4.7.5` otherwise I was getting vague compatibility errors between minitest and test/unit.

Feel free to cherry pick. Thanks! :)