behat: separate tests that require the `ronn` gem

Currently, we call `gem install ronn` on each test run, which takes over 30 seconds and is only used in a handful of tests.

So, I think we should use the following technique: http://about.travis-ci.org/blog/2012-11-28-speeding-up-your-tests-by-parallelizing-them/

and using Behat filters: http://docs.behat.org/guides/6.cli.html#gherkin-filters