Fix JSONField deprecation warning

webapp/models.py

@@ -11,7 +11,7 @@ import requests
 from uuid import uuid4
 import logging
 from django.utils.translation import ugettext_lazy as _
-from django.contrib.postgres.fields import JSONField
+from django.db.models import JSONField
 from auditlog.registry import auditlog
 from polymorphic.models import PolymorphicModel
 from webapp.utils import month_from_now, send_signup_email