Improve python version-specific tests in CI

[jooh]

The bindit tests call the bindit cli, and this seemed enough to confuse travis. I hacked my way around it by explicitly installing the package before triggering tox:

bindit/.travis.yml

Line 8 in 63f0398

- python setup.py -q install

But I think this may result in the same python version getting used inside the bindit CLI. Need to do more testing to confirm this issue and work around it. For now it doesn't much matter since python 3.6 and 3.7 are so similar (and we don't support anything older).