Bump a few requirements to increase compatibility

The upgraded packages all increase Python 3 compatibility. Bumping was
either necessary because the current release doesn't support Python 3,
or at least advisable.
Use unreleased versions of Compressor and Sorl for Python 3

Let's hope both of them release a new stable version in time for Oscar
0.7.
Introduce requirements_py2.txt for Python 2-only dependencies

Unfortunately detox and transifex-client don't even install under Python
3. Moving them to a seperate file will help with Travis CI.
Drop requirements_vagrant.txt

Only contains database bindings. They belong into an Oscar's developers
toolbelt anyway, and hence into requirements.txt.
Update Travis CI and tox to run against Python 3
Skip MySQL migration tests for Python 3

Django doesn't recommend a package for MySQL bindings that works with
both Python 2 and 3, so it seems best to just skip those tests for now.