Drops Python 3.3 from .travis.yml.

Pip no longer supports Python 3.3:
https://github.com/pypa/pip/issues/3796

Python 3.3's end of life was roughly a year ago anyway, so
this seems like a good time to drop it from the test matrix:
https://www.python.org/dev/peps/pep-0398/#x-end-of-life
1 file changed