FIX Discount->getSavingsForOrder sql to be postgres compatible
FIX Discount->getSavingsForOrder sql to be postgres compatible

burnbright/silverstripe-shop-discount,burnbright/silverstripe-shop-discount,markguinn/silverstripe-shop-discount,markguinn/silverstripe-shop-discount,IgorNadj/silverstripe-shop-discount,IgorNadj/silverstripe-shop-discount