Update test suite to rely on the Minitest gem

Currently the test suite cannot be run on a local machine with a modern Ruby version (I'm on 2.1). 

This updates the suite to rely on the minitest gem. This maintains compatibility against 1.9.3+.

/cc @bkeepers @github/user-content 
I only knew because of http://yaml.travis-ci.org/.

I tried last night to figure out why the CI UI connection between GitHub and Travis has stopped for this repo. And I can't figure it out!