Depend on generic Java 7 runtime instead of OpenJDK7

Instead of depending on the specific `openjdk-7-jre-headless` this Debian package should depend on the virtual package `java7-runtime-headless`.

For example we're using a custom built Debian package for Oracle JDK 7 in our setup which provides `java7-runtime-headless` and we do not want to switch to OpenJDK 7 at the moment.