Update Travis xvfb configuration (#501)
Update Travis xvfb configuration (#501)

Travis CI released an update to their Xenial build environemnt, which
introduced a new way to start up XVFB for front-end builds. You now
simply add

    services:
      - xvfb

to your Travis shard config instead.

See: https://changelog.travis-ci.com/ubuntu-xenial-16-04-build-environment-update-86123
google/quiver-dart,cbracken/quiver-dart,cbracken/quiver-dart,kevmoo/quiver-dart,kevmoo/quiver-dart