Use Minitest to run sass-spec

By using Minitest we can create custom reporters so the output can be consumed by other applications. Also, Minitest runs tests in parallel. This speeds up the full test suite by 2x on my machine.

This is the finished refactor mentioned in #27.
It's more that I didn't see an easy way to replicate it. I suppose we could monkey patch minitest or make a custom reporter, but the feature didn't seem very useful so I just commented it out. Do you use it?