Touch shipment when updating shipment rate

Fixes solidusio#1909
If shipment rate is updated it does not touch shipments, and shipments
wont touch order, leading to a out of sync cached order

core/app/models/spree/shipping_rate.rb

@@ -3,7 +3,7 @@ module Spree
  # method has been selected to deliver the shipment.
  #
  class ShippingRate < Spree::Base
- belongs_to :shipment, class_name: 'Spree::Shipment'
+ belongs_to :shipment, class_name: 'Spree::Shipment', touch: true
  belongs_to :shipping_method, -> { with_deleted }, class_name: 'Spree::ShippingMethod', inverse_of: :shipping_rates
 
  has_many :taxes,