! Use newly added MiniTest::Spec::DSL

New direction for minitest-rails.
Instead of managing the ancestry to AS::TC, extend it with MT::S::DSL instead.
Allows for most of the complexity of minitest-rails to go away,
and we can focus on more on smoothing edges than maintaining hacks.
Thanks @zenspider!
Update minitest dependency

Per Ryan's request we'll stay on the 4.7+ version for now.