Replace cucumber in grunt tests with cheapseats
Remove Cucumber and feature tests

- Stop not linting the features directory
- Remove Cucumber from the Gruntfile and package.json
- Update the README to reflect cheapseats
- Remove config related to Cucumber tests
- Remove feature tests

Our feature tests have moved from Cucumber to JavaScript, so the
tests themselves are no longer in this app.
Purge Ruby

- Stop installing gems on Travis
- Remove Gemfile
- Update README, Jenkins script and start-app.sh to remove bundle install
  step
Merge pull request #345 from alphagov/feature/cheapseats-integration

Replace cucumber in grunt tests with cheapseats