Check for SHIPPABLE, not JENKINS

Shippable sets this environment variable [0] so we can use it to check
whether we’re running tests there. Travis doesn’t need these settings so
we don’t want to check for a more general variable (e.g.
CONTINUOUS_INTEGRATION) that would be set on both services.
Add `bundle exec` to Shippable script
Get rid of Travis

Nobody’s using it, so it’s just confusing to have the .travis.yml file
and build status badge.
Merge pull request #214 from ministryofjustice/fix-ci-on-develop

Fix CI