Start xvfb using 'services' rather than a run script. (#13)
Start xvfb using 'services' rather than a run script. (#13)

This is necessary now that Travis has switched from Trusty to Xenial by
default.

(See https://docs.travis-ci.com/user/gui-and-headless-browsers/#using-services)
bazelbuild/vim-bazel,bazelbuild/vim-bazel,bazelbuild/vim-bazel