transmission: security bump to version 2.84

Fixes CVE-2014-4909 (peer communication vulnerability).

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Acked-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>