Cleanup `tox.ini`
Run tests against several Swift versions
Only run `py27-swift2.2.0` by default
Fixup `pep8` tox target
Update `travis.yml` with new Tox targets
Don't complicate things by parametrizing Python version

Just use Python 2.7 for everything.
Allow running tests against Swift `head`
Fix `cover` Tox target
Add `pylint` Tox target
Bump tox min version to 1.8.1
Merge pull request #10 from scality/test-multi-version

Enable testing against several Swift versions