[WIP] Compatibility with Rails 4.2

* add some testing dependencies to gemfile to avoid conflicts with system minitest
* fix errors for everything running against Rails 4.2.0.beta1 except most associations

This is still a work in progress. Most of the tests were ending in error or failing, but now only a couple dozen are bad. All the remaining failures/errors all related to assocations.

I'll try to spend some more time during the week this week to finish it off, but there are no guarantees I'll be able to make time, so I figured it would be good to post this work-in-progress in case anyone else can work on it with a head start and not have to duplicate the efforts I put in today.