Run TravisCI builds in two parallel processes, one for unit tests and another for acceptance tests, using the TravisCI build matrix feature.

Parallel build inspired by this blog post:

  http://about.travis-ci.org/blog/2012-11-28-speeding-up-your-tests-by-parallelizing-them/

Rake task to selectively run Rspec specs depending on an environment variable inspired by this blog post:

  http://murphyslaw.github.io/blog/2012/04/06/run-specs-excluding-integration-specs/