Use minitest directly

Since Ruby 1.9, `test/unit` has just been a wrapper around minitest for backwards compatability:

http://www.rubyinside.com/a-minitestspec-tutorial-elegant-spec-style-testing-that-comes-with-ruby-5354.html

The driver for this change is that I want to add support the Ruby Best Practices gem which has the gem version of minitest as a dependency. Mixing `test/unit` and `minitest` together causes problems.