Orders with the since_id parameter are now sorted by order ID

Prior to September 18, 2017, requests to GET /admin/orders.json including the since_id parameter would return a list of orders sorted by created_at. This would present problems when multiple orders were created within the same second, as results wouldn't dependably return in the same order on subsequent requests. Going forward, orders with the since_id parameter will correctly be sorted by order ID, even if the orders were processed at identical created_at times.