Update some configuration

Just some polishing on the gemspec and spec_helper. The commits all explain what they're doing and why. The main benefit of this is in the last commit, which will allow us to play with [mutant](https://github.com/mbj/mutant) to test our test suite with mutation testing, which is something I'm interested in. For some reason, without the configure block mutant wasn't picking up `describe` and raising errors.