Implement only change suggested by 2to3.
Python 3 compatibility: Ensure check_output returns a string, not a bytes obect.
Python 3 compatibility: `OSError` became `FileNotFoundError`. 
This supports both variants (and the errno is the essential info, anyway)
Add working setup for tox (http://tox.readthedocs.org/en/latest/)
Make Travis use tox.