zilencer: Add on_delete in foreign keys.

on_delete will be a required arg for ForeignKey in Django 2.0. Set it
to models.CASCADE on models and in existing migrations if you want to
maintain the current default behavior.
See https://docs.djangoproject.com/en/1.11/ref/models/fields/#django.db.models.ForeignKey.on_delete

zilencer/migrations/0002_remote_zulip_server.py

@@ -2,6 +2,7 @@
 from __future__ import unicode_literals
 
 from django.db import migrations, models
+import django.db.models.deletion
 
 
 class Migration(migrations.Migration):
@@ -36,6 +37,6 @@ class Migration(migrations.Migration):
         migrations.AddField(
             model_name='remotepushdevicetoken',
             name='server',
-            field=models.ForeignKey(to='zilencer.RemoteZulipServer'),
+            field=models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, to='zilencer.RemoteZulipServer'),
         ),
     ]