[INTERNAL] made packet extensions incompatible

Needed for next release to ensure that Saros is not able to communicate
with outdated clients.

Change-Id: Idb6c5afafaac85fa2806aaf46da11e692b727f83
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1175
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>