drop -XX:MaxPermSize=384m from jenkins' java args

From the Jenkins log:

OpenJDK 64-Bit Server VM warning: ignoring option MaxPermSize=384m;
support was removed in 8.0

Signed-off-by: Mattia Rizzolo <mattia@debian.org>
Signed-off-by: Holger Levsen <holger@layer-acht.org>