Remove unneeded Xvfb from Travis config
Remove unneeded Xvfb from Travis config

Remove the "xvfb" service. This is no longer used since browser
testing was moved to SauceLabs. Also, for Firefox Headless or
PhantomJS, Xvfb wouldn't be needed (mainly for pre-headless era
of browser testing.)

jashkenas/underscore,documentcloud/underscore,jashkenas/underscore,documentcloud/underscore