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.
my bad, it does.  i could've sworn it didn't. :)

(the "id asc" was intentional though, since i called ".last". rails transforms that automatically into "id desc" when .last is called).

closing the PR.