Get tests working on Travis for Drupal 8 in a minimal way

I'd really love to port all the Drupal 7 tests to Drupal 8 and implement test sharing between versions in some way, however, that's a really big task and at the moment we just need tests working at all :-)

See #2495739: Evaluate custom Behat steps and move them upstream for thoughts on how we can reach an ideal state of a slimmed down panopoly_test module and code sharing