Support for parallel_rspec

Support parallel_rspec for faster tests.

I don't personally care if it's enabled on CI or not, just that I have an option to use it locally to speed up testing. In other words, I just care that the tests are isolated so they are [deterministic even when run in parallel](http://martinfowler.com/articles/nonDeterminism.html#LackOfIsolation).

However, I have enabled it on Travis to get a benchmark and because @joshk expressed interest.