Travis use tox

This reduces duplication between `tox.ini` and `.travis.yml` by having `.travis.yml` leverage the tox configuration.

I also fixed the tox "cover" target which was broken for me on nosetests 1.3.0 and made it run by default so we can track our coverage and add more tests to improve coverage which will aid the python 3 port.

See https://travis-ci.org/msabramo/supervisor/jobs/22947384 for a passing Travis CI build with coverage report

Cc: @mnaberez, @mcdonc, @mmerickel