Add TravisCI config

In preperation for enabling TravisCI to run lint and unit tests against our
pull requests. This:

- Uses the Ruby defaults such as `bundle install` and `bundle exec rake`.
- Only tests 1.9.3 at the moment, which I think is what we're targetting?
  But we might want to extend it to 2.x in the future.
- Doesn't test against master because that is handled by our in-house CI
  system which also runs integration tests.
- Disables email notifications because we already get a lot of email and the
  PR status hook on GitHub should be good enough.
Merge pull request #23 from alphagov/70499778-add_travis_config

[#70499778 ] Add TravisCI config