.travis.yml: Use tox so more DRY

This makes the .travis.yml use tox so there is less repeated code.
Fix tox "cover" target and run by default

Previously, tox "cover" target failed for me on nosetests 1.3.0 with:

    nose.plugins.xcover: ERROR: You can not use both --with-xcover and
    --with-coverage. Using --with-xcover implies --with-coverage

because there was a confusing mix of nose options in both setup.cfg and
tox.ini.

I moved the coverage-related stuff from setup.cfg to tox.ini and now the
tox "cover" target works for me with nosetests 1.3.0
Merge pull request #415 from msabramo/travis_use_tox

Travis use tox
Remove unused test_requirements.txt
This file was only referenced in .travis.yml and that reference was
removed in 52258069b7058fa1800177c9f33d3c699a95ef79.