Return non-floating ref from IBusConfigGConf::get_value

Both IBusConfigDConf::get_value and IBusConfigMemconf::get_value
return a non-floating reference, so we need to be consistent in the
gconf backend and also return a non-floating reference.

BUG=http://code.google.com/p/ibus/issues/detail?id=1712

Review URL: https://codereview.appspot.com/106830043
Patch from Christophe Fergeau <cfergeau@redhat.com>.