Use activesupport, update to latest Minitest/specs

Active Support will be useful for its inflector for a different task, which will branch off of this (as well as for other things!). As a result, however, using it as a runtime dependency shares common dependencies with Minitest. I had to upgrade Minitest, which made a number of breaking changes, and as a result I needed to make all the tests satisfy Minitest's new conventions.

Therefore I decided to just get rid of `shoulda` and migrate our tests over to `Minitest::Spec` format to bring homogeneity with the testing style in other Clever codebases. Schweet