[DEB] Depend on java-runtime-headless

On Debian Java packages consist from two parts, regular one with graphical dependencies and -headless part, which is suitable for servers and applications like logstash or elasticsearch.

Moreover default-jre on Wheezy depends on openjdk-6-jre. While java-runtime-headless is provided by both openjdk-6-jre-headless and openjdk-7-jre-headless.