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