Revise install instructions to use wxPython 2.9.5.0, plus needed patch file
Minor cleanup of INSTALL.txt
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
Update .travis.yml: remove Python3 test environments

For example:
- https://travis-ci.org/BruceSherwood/vpython-wx/jobs/12460551
- https://travis-ci.org/BruceSherwood/vpython-wx/jobs/12460552
Merge pull request #35 from glarrain/patch-2

Update .travis.yml: remove Python3 test environments
Merge pull request #34 from glarrain/patch-1

Update .travis.yml: added 'xvfb' to imitate a monitor
Add info about building wxPython 2.9.5 on 32-bit Linux; correct typo in docs
Minor tweaks to window management on Linux

The bugs (at least on Ubuntu) were that initially a window had a white bar at the bottom, and resizing a window drove an unneeded _OnMove routine with the result that resizing didn't do the right thing.