Mark migrations with data operations "non atomic"

Ref: https://docs.djangoproject.com/en/dev/howto/writing-migrations/#non-atomic-migrations

InvenTree/build/migrations/0013_auto_20200425_0507.py

@@ -17,6 +17,8 @@ def nupdate_tree(apps, schema_editor):
 
 class Migration(migrations.Migration):
 
+    atomic = False
+
     dependencies = [
         ('build', '0012_build_sales_order'),
     ]

InvenTree/build/migrations/0018_build_reference.py

@@ -31,6 +31,8 @@ def reverse_default_reference(apps, schema_editor):
 
 class Migration(migrations.Migration):
 
+    atomic = False
+
     dependencies = [
         ('build', '0017_auto_20200426_0612'),
     ]