gravatar: Use new register_keys vfunc
gravatar: Use new register_keys vfunc

https://bugzilla.gnome.org/show_bug.cgi?id=740943

MathieuDuponchelle/grilo-plugins,MikePetullo/grilo-plugins,MathieuDuponchelle/grilo-plugins,jasuarez/grilo-plugins,MathieuDuponchelle/grilo-plugins,MathieuDuponchelle/grilo-plugins,grilofw/grilo-plugins,grilofw/grilo-plugins,GNOME/grilo-plugins,MikePetullo/grilo-plugins,jasuarez/grilo-plugins,GNOME/grilo-plugins,MikePetullo/grilo-plugins