Refactor price db structure

#475

db/migrate/20170423210622_change_price_price_cents_type_to_integer.rb

@@ -0,0 +1,5 @@
+class ChangePricePriceCentsTypeToInteger < ActiveRecord::Migration
+  def change
+    change_column :prices, :price_cents, 'integer USING CAST(price_cents AS integer)'
+  end
+end

db/migrate/20170423214500_remove_price_price_currency.rb

@@ -0,0 +1,5 @@
+class RemovePricePriceCurrency < ActiveRecord::Migration
+  def change
+    remove_column :prices, :price_currency, :string
+  end
+end

db/migrate/20170423222302_remove_price_price_cents_default.rb

@@ -0,0 +1,5 @@
+class RemovePricePriceCentsDefault < ActiveRecord::Migration
+  def change
+    change_column_default :prices, :price_cents, nil
+  end
+end