Revert "Upgrading to Minitest 5.2"

This reverts commit 06ff6df6a3777f2088629d855b82df8303208c73.

Rails 4 has a dependency on MiniTest 4.x... so getting to 5.2 is more pain than
it's worth at the moment. Reverting the upgrade until it's possible to do so
without annoyance.