Split sandbox/demo make targets so installation can run separately

This is to avoid an issue on Travis where Django 1.7 is installed when
it shouldn't be.
Remove basket FK

As this breaks the fixture loading process. Should never have been there
in the first place.
Re-enable allowed failures in Travis for Django 1.6 and Python 3

Turns out I was wrong to remove them...
Change database name to oscar_travis for migration testing

oscar_vagrant was confusing.