Revert "Removed DEFAULT_AUTO_FIELD setting."

This reverts commit 162ffe9.

Author: tobiasmcnulty

settings.py

@@ -57,6 +57,8 @@ def _build_databases_setting():
 
 DATABASES = _build_databases_setting()
 
+DEFAULT_AUTO_FIELD = "django.db.models.AutoField"
+
 PASSWORD_HASHERS = ["django.contrib.auth.hashers.MD5PasswordHasher"]
 
 SECRET_KEY = "django_tests_secret_key"