gajim: Update gajim to version 0.15.4.

This fixes the following bugs:

 * traceback on decryption of OpenPGP-encrypted messages
 * remote_control: use GED in send_chat_message
 * gajim-0.15.3 can't connect to server without StartTLS support
 * "make check" fails with complaining on src/command_system/mapping.py
 * Account Wizard: Make text selectable
 * Gajim crashes in connect.py

Overview and details:

https://trac.gajim.org/query?status=closed&milestone=0.15.4



Signed-off-by: aszlig <aszlig@redmoonstudios.org>