add rails integration test for api authentication
remove api authentication cucumber feature

there is an equivalent rails integration test for this
removing unnecessary .gitkeep
api auth test: 'should' is more expressive than 'test'
api auth test: use rack-test instead of capybara
Merge pull request #140 from alphagov/port-api-authentication-test-from-cucumber-to-rails-integration

Port api authentication test from cucumber to rails integration