Add run_tests.sh and make travis.yml explicit

- We use `run_tests.sh` elsewhere so it's consistent.
- On my machine, when `bundle exec rake` fails it does so with no
  output, so it's not at all clear that it has failed!
- It's weird that `.travis.yml` magically runs rake because it's a
  ruby project - better to be explicit.
Merge pull request #322 from alphagov/explicit-test-runner

Add run_tests.sh and make travis.yml explicit