Fix wrong discount calculation with flat percent promotions

Fix wrong discount calculation with flat percent promotions when there are more than one line item in the order.

This error happened because the order instance here:
https://github.com/spree/spree/blob/master/core/app/models/spree/order_updater.rb#L24

is not always the same instance in memory here:
https://github.com/spree/spree/blob/master/core/app/models/spree/calculator/flat_percent_item_total.rb#L15

Adding the inverse option to the relationship makes sure you have the same object instance in both places.