Use delivery instead of express in product

db/migrate/1_create_shotengai_products_and_orders.rb

@@ -11,7 +11,7 @@ class CreateShotengaiProductsAndOrders < ActiveRecord::Migration[5.1]
       t.string :title
       t.string :status
       t.integer :default_series_id
-      t.integer :express_way
+      t.integer :delivery_cost
       t.boolean :need_time_attr
       t.string :cover_image
       t.json :banners

lib/shotengai/product.rb

@@ -70,7 +70,7 @@ module Shotengai
       }[ status.to_sym ]
     end
 
-    enum express_way: {
+    enum delivery_way: {
       no_need: 0,
       delivery: 1,
       pick_up: 2,