Updater (cmd-line): avoid DBus messages in headless mode

When running the updater in Jenkins (i.e. headlessly, on Linux),
there is no good reason to try to figure out the system proxy
settings (because they are not available it is bound to X11 on
Linux...).

Incidentally, avoiiding that avoids these pesky messages:

	GConf-WARNING **: Client failed to connect to the D-BUS daemon:
	Unable to autolaunch a dbus-daemon without a $DISPLAY for X11
	GConf Error: No D-BUS daemon running

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>