[code-quality] Flake8 pre-commit hook in virtualenvs and vagrant

If you use a tool like SourceTree, having the first line as `#!/usr/bin/env python` in your pre-commit script will mean that it doesn’t work if you’re using a virtualenv. Instead you need to change the first like to something like `#!/usr/local/virtualenvs/myproject/bin/python`. I actually document that in the docs to my Django Base Site (http://django-base-site.readthedocs.io/en/latest/pre-commit-hook.html). So the question is, should that be added to the flake8 documentation (http://flake8.readthedocs.io/en/latest/vcs.html)? Also, has anyone found a way to make tools like SourceTree work on your host system, when your project is using a virtualenv under a vagrant VM? If so, could this get added to the docs as well? - Brent