.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