Reluctantly use MiniTest 4 :/

It is with great sadness and a certain amount of annoyance that I'm
finally succumbing to the pressures and using MiniTest 4, in order to be
able to upgrade activerecord.
Call .connection on model class.

Calling ActiveRecord::Base.connection, or calling #connection on an
instance is deprecated in Active Record 4.
Explicitly mute submissions in tests
Upgrade activerecord