Update .travis.yml: use xvfb according to travis docs
Update .travis.yml: use xvfb according to travis docs

See https://docs.travis-ci.com/user/gui-and-headless-browsers/#using-xvfb-to-run-tests-that-require-a-gui
samdauwe/BabylonCpp,samdauwe/BabylonCpp,samdauwe/BabylonCpp,samdauwe/BabylonCpp