ar_orderby has changed to qb_orderby

@Narf

Thanks for the heads-up on this one and I've changed my code as per. However, methinks you might get a few queries on this one when people move to CI 3 as this "not so little gem" was one of the tricks published on "Codeigniter learning series" by Nettuts.

Most things broken in our house was done by nobody, the rest was my fault anyway