Code review comment for lp:~nskaggs/update-manager/fix-for-702418

Thanks for your branch!

Testing for gnome in the env is not quite sufficient as the default session will use
"XDG_CURRENT_DESKTOP=Unity" and DESKTOP_SESSION=ubuntu these days. Actually this makes
me wonder if calling the old interface can be dropped entirely? Could you maybe check
if gnome now supports the freedesktop one?