Run RSpec tests along with minitest

I like [the idea](https://github.com/caskroom/homebrew-cask/commit/58d4c745a9e160305236c3a561874035a3909660) of gradually rewriting tests to rspec, but we should also make sure to keep the new tests green and this change should help everyone to do so.

In fact [this change](https://github.com/caskroom/homebrew-cask/commit/db0b5aa9d9af6a8917508c4757bb26a0bf07253a) already caused 4 rspec tests to fail.

cc @phinze @rolandwalker