[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`. By default `openjdk-6-jre-headless` has higher priority so it will be installed. This allows to keep number of installed java packages at minimum.