Update external dependency: system-tools-backends

[Carlos Garnacho <carlosg gnome org>]

Hi all!,

I've just released system-tools-backends 2.4.0, this release was
intended to be synchronized with the 2.20 schedule, and I'd like it to
be the recommended version, The minimum version can stay as it is, as
they're compatible.

Reasons? Lots of bugfixing, and being able to set WPA in network-admin
relies on it.

Regards,
  Carlos