for testing node-webkit, launch a virtual display

xvfb recipe:
http://docs.travis-ci.com/user/gui-and-headless-browsers/#Using-xvfb-to-Run-Tests-That-Require-GUI-(e.g.-a-Web-browser)

sleep 3 recipe:
http://docs.travis-ci.com/user/gui-and-headless-browsers/#Starting-a-Web-Server