Removes dependency on old Spree::Order payment method.

Replaces dependency on the old payment method copied from Spree::Order
model with calls to the state aware pending_payments method from
Spree::Order. Only applies to the first pending payment to ensure
credit is not applied multiple times on same order.
Merge branch '1-3-stable' of github.com:spree/spree_store_credits into 1-3-stable