gb: move "giffgaff" after "O2"

Both providers have the same mcc/mnc. As O2 is more common,
move it first.

https://bugzilla.gnome.org/show_bug.cgi?id=750330
https://bugs.launchpad.net/ubuntu/+source/mobile-broadband-provider-info/+bug/1253586



Reported-by: Alex J Lennon <ajlennon@dynamicdevices.co.uk>