Fix compatability for south

Introduction of `CustomForeignKey` in [1.4.0](https://github.com/treyhunner/django-simple-history/tree/1.4.0) broke south migrations as well as the ([already fixed](https://github.com/treyhunner/django-simple-history/pull/128)) 1.7 migrations. This should get south migrations working as well.

Fixes #129 

@mghantous, it would be awesome if you could verify that these changes get south working again before we merge it into master.