Update orders.promo_total in OrderUpdater

NOTE: Cherry-picked from this spree pull request until it gets officially resolved: https://github.com/spree/spree/pull/4607

orders.promo_total field wasn't getting updated in certain cases, like when you add the promo first and then a line item afterward.
the *adjustment* got updated and the correct number was displayed in the cart, but the orders.promo_total value was incorrect.