Fixes Spree::Subscription#last_order

The method was behaving incorrectly in two ways:

- associated Spree::Order records were being ordered by `completed_at DESC` *in addition* to any existing ordering done, rather than exclusively.
- the method was selecting the *last* order from the complete orders collection, which, due to sorting by `completed_at` in *descending* order, would always be the *first* order chronologically speaking.