Tests were failing on postgres due to foreign keys pointing to missing
tables.
In order for these tables to be present, we need migrations which
declare depending on `content_type`.
This avoids the following error in django 1.11 tests:
polymorphic.MRODerived: (models.E005) The field 'id' from parent model 'polymorphic.mrobase3' clashes with the field 'id' from parent model 'polymorphic.mrobase1'.
Related to https://code.djangoproject.com/ticket/22442