fix(add notification number): add explicit default

there is a possibility that this will not work with postgres

Issue: https://www.pivotaltracker.com/story/show/144646649

- [ ] It works!
- [ ] Comments provide sufficient explanations for the next contributor
- [ ] Tests cover changes and corner cases
- [ ] Follows Quay syntax patterns and format

data/migrations/versions/dc4af11a5f90_add_notification_number_of_failures_.py

@@ -15,7 +15,10 @@ from alembic import op
 
 
 def upgrade(tables):
-  op.add_column('repositorynotification', sa.Column('number_of_failures', sa.Integer(), nullable=False))
+  op.add_column('repositorynotification', sa.Column('number_of_failures',
+                                                    sa.Integer(),
+                                                    nullable=False,
+                                                    server_default='0'))
   op.bulk_insert(tables.logentrykind, [
     {'name': 'reset_repo_notification'},
   ])