Rename Delivery.delivered_on to date

This aligns it with column names of StockTaking and us to use inheritance
in a next step to share common code between the entities.

db/migrate/20181204060000_change_delivered_on_to_date.rb

@@ -0,0 +1,5 @@
+class ChangeDeliveredOnToDate < ActiveRecord::Migration
+  def change
+    rename_column :deliveries, :delivered_on, :date
+  end
+end