Make .travis.yml use tox
Make .travis.yml use tox

Makes it more DRY and can see individual Travis jobs for each Python
version

ezequielo/diff-cover,mblayman/diff-cover,benpatterson/diff-cover,Bachmann1234/diff-cover,mblayman/diff-cover,Bachmann1234/diff-cover,hugovk/diff-cover,ezequielo/diff-cover,benpatterson/diff-cover,edx/diff-cover