Fixed missing optional argument added in DRF 2.3.14

DRF 2.3.14 adds an optional argument to `get_validation_exlusions`:
https://github.com/tomchristie/django-rest-framework/commit/51e6982397cc032d6b3fd66f452713d448eb9084

We override that method to ignore the user FK which we add ourselves.

Fixes #7.