Add migration for Document.documentproperties ref

This should have been included with a8dcbbb, which changed the
related_name to access DocumentProperties from Document. (There's no
actual change to the database.)

pandora/entity/migrations/0004_related_name_Document_documentproperties.py

@@ -0,0 +1,21 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.9.4 on 2016-04-21 06:23
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('entity', '0003_documentproperties_data'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='documentproperties',
+            name='document',
+            field=models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, related_name='documentproperties', to='document.Document'),
+        ),
+    ]