ensure correct order for payments query in OrderUpdater

".last" without a sort order is not guaranteed to return any particular item, especially in postgres.

"If sorting is not chosen, the rows will be returned in an unspecified order"
http://www.postgresql.org/docs/9.3/interactive/queries-order.html

I couldn't think of a reasonable way to write a spec around this since the sort order is just unspecified.
wondering if rails already doesn't add that `order`, also did you mean `id DESC` here?