Fix migrations for statsmetadata model

Remove manually added 'default=None' that is not needed.

Change-Id: I31ed3e23bb634a7d5c2f2e69c533498815e3491d
diff --git a/crashreport_stats/migrations/0005_remove_manual_default_value.py b/crashreport_stats/migrations/0005_remove_manual_default_value.py
new file mode 100644
index 0000000..3c508ed
--- /dev/null
+++ b/crashreport_stats/migrations/0005_remove_manual_default_value.py
@@ -0,0 +1,22 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.10.2 on 2018-08-16 09:10
+"""Remove previously added default value for updated_at column."""
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    """Remove default None value which was added manually."""
+
+    dependencies = [
+        ('crashreport_stats', '0004_statsmedata_and_no_broken_default_dates'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='statsmetadata',
+            name='updated_at',
+            field=models.DateTimeField(),
+        ),
+    ]