.travis.yml: Bring over changes from 1.0-alpha branch

These are the changes that @sigmavirus24 did to `.travis.yml` on the
`1.0-alpha` branch in sigmavirus24#287
tox.ini: Use py.test directly

instead of `python setup.py test`.

The advantage of using `py.test` directly is that you can now pass
`py.test` options to take advantage of all of its neat features for
controlling test collection, output control, pdb, etc. E.g.:

    tox -e py27 -- --tb=short --pdb --pyargs tests.integration.test_repos_release -k test_download