migrations: Correct 'unique_together' order in '0015'

migrations: Correct 'unique_together' order in '0015'

This was resulting in exceptions like the following when used with MySQL
8.0:

  Traceback (most recent call last):
    File "../patchwork/manage.py", line 11, in <module>
      execute_from_command_line(sys.argv)
    ...
    File "../.tox/py27-django111/lib/python2.7/site-packages/django/db/backends/mysql/schema.py", line 88, in _delete_composed_index
      return super(DatabaseSchemaEditor, self)._delete_composed_index(model, fields, *args)
    File "../.tox/py27-django111/lib/python2.7/site-packages/django/db/backends/base/schema.py", line 394, in _delete_composed_index
      ", ".join(columns),
  ValueError: Found wrong number (0) of constraints for patchwork_seriespatch(series_id, number)

This error was being raised by the following lines in the 0033
migration:

  migrations.AlterUniqueTogether(
      name='seriespatch',
      unique_together=set([]),
  )

It appears that this is because of a mismatch between the order of
fields in a 'unique_together' constraint [1]. Correct the order in the
original migration and see the issue disappear.

Signed-off-by: Stephen Finucane <stephen@that.guru>
Fixes: d67d859f40f ("models: Add 'Series' model")