Fix parent error when deleting category

- Any child categories have their parent set to the parent of the deleted category

InvenTree/stock/migrations/0005_auto_20180415_0107.py

@@ -0,0 +1,21 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11 on 2018-04-15 01:07
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('stock', '0004_auto_20180414_1032'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='stocklocation',
+            name='parent',
+            field=models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.DO_NOTHING, related_name='children', to='stock.StockLocation'),
+        ),
+    ]