lock minitest to unlock the green color of joy for tests

I don't know what do you folks like more - hanging around with `Test::Unit` or `Minitest`, so I've just locked the `Minitest` version to something lower than 5.0.

[Anything newer than `Minitest` 5.0 is incompatible with `Test::Unit`](https://github.com/seattlerb/minitest/issues/283).

This should make the Travis pass again, but I'm not sure. It made my local Travis happy.