update sub-repo location

(`"github.com/xtgo/slug"` is the ideal path for this package, as it's a much shorter import path, and it's where @extemporalgenome's stable Go libraries live. Thus, I'm merging the @robfig's PR here instead)

As `"code.google.com/go.text/unicode/norm"` now _depends_ on `"golang.org/x/text/unicode/norm"`, and the latter uses capacity setting syntax introduced with 1.3, direct backwards compatibility with 1.2 and earlier is effectively outside of our control. If anyone needs 1.2 or earlier support, they'd need to fetch an earlier revision of the code.google.com/p/go.text repo anyway, so it wouldn't be out of their way to fetch an earlier rev of this repo as well.