Embed hspec-expectations-pretty-diff into the repo and remove pinned dependency.

[patrickt]

We can’t pin this dependency if we want to upload to Hackage. Unfortunately, it produces worse-looking but more-informative output than just using hspec-expectations on its own. Because the output is flawed, I don’t think we should put it on Hackage—it’s a tiny library, let’s just pull it into the test suite.

Blocking #16.

[patrickt]

Closing in favor of #120.