Update .travis.yml: added 'xvfb' to imitate a monitor

Travis is still unable to complete all steps. Since it uses an OS without "desktop", it has no display for wxPython to access.

https://travis-ci.org/BruceSherwood/vpython-wx/jobs/12460550
Merge pull request #34 from glarrain/patch-1

Update .travis.yml: added 'xvfb' to imitate a monitor