Use Travis matrix to ensure Python interpreters are installed

Master is currently failing with tox on versions of Python != 2.7.  I suspect that something changed about how they install Python interpreters on the images.

This PR specifies Python versions in the Travis YML file instead of relying on tox.  This seems to fix the build in Travis.

Unfortunately, I'm getting unicode errors when `diff-cover` runs on its own coverage report in Python 3.3.  I'm not sure why the test suite didn't catch this before; for now, I'm going to disable the 3.3 tests to get master back to green.

@singingwolfboy