Django 2.0+ compatibility

[browniebroke]

Fixes #24

• [x] Add on_delete=models.CASCADE to all foreign keys (including migrations)
• [x] Update Django 2.1a1 to 2.1 in tox.ini
• [x] Update classifiers
• [x] Remove Django 2.0 and 2.1 from the allowed failures

[kennydude]

@melbic Could you have a look into this? Scarface is one of the few things holding us back from moving our projects to Django 2

[melbic]

Sure, sorry I did no know it prevented folk from upgrading. Sorry.