3421: Add XVFB to travis so that it can run tests in firefox
3421: Add XVFB to travis so that it can run tests in firefox

Authors: 7822e016f35d7b932307e38a43c76bdf8d3c8ebe@davbo

alphagov/verify-frontend,alphagov/verify-frontend,alphagov/verify-frontend,alphagov/verify-frontend