Discount code uses_remaining is never decremented

[molokov]

Small bug in Order.complete():
https://github.com/stephenmcd/cartridge/blob/master/cartridge/shop/models.py#L474-L501

The fields from self.session_fields are deleted at the top of the function, but we don't get the discount_code out of the session until later in the function - and discount code is one of those deleted as it's listed in self.session_fields.

This way, the uses_remaining field is never decremented when the discount code is used, as it will always be None.

Simple enough to fix by some rearrangement of lines in the function.