Update DB migrations following upstream change in ICEkit

The `WorkflowStateMixin` model in django-icekit -- which
is used as a basis for the `EventBase` model -- was updated
with two new fields: `brief`, and `admin_notes`.

This change updates the model in this project to comply
with the upstream changes.

And will hopefully make ICEkit unit tests pass again in Travis.

icekit_events/migrations/0025_auto_20170519_1327.py

@@ -0,0 +1,24 @@
+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('icekit_events', '0024_auto_20170320_1824'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='eventbase',
+            name='admin_notes',
+            field=models.TextField(help_text=b"Administrator's notes about this content", blank=True),
+        ),
+        migrations.AddField(
+            model_name='eventbase',
+            name='brief',
+            field=models.TextField(help_text=b'A document brief describing the purpose of this content', blank=True),
+        ),
+    ]