Refactor spree_api:

* Use find_by() instead of dynamic find_by_something()
* Minor syntax cleanup
Refactor spree_core:

* Use Rails4 find_by()
Pass updated order object to `adjustment.update!`

So that the adjustment can be recalculated over updated order totals. It
also introduces an `update_shipping_adjustments` method because we
shipping adjustments will need a shipment object and in that scenario we
don't need to pass the order