Update .travis.yml

Add xvfb (X Virtual Framebuffer) virtual screen support for future testing.
According to official Docs at:
http://about.travis-ci.org/docs/user/gui-and-headless-browsers/#Using-xvfb-to-Run-Tests-That-Require-GUI-(e.g.-a-Web-browser)
Merge pull request #1452 from juancarlospaco/patch-1

Update .travis.yml