Ubuntu/Debian should depend on 'java-runtime-headless'

This fixes #893. The previous dependency "default-jre-headless" is just
a metapackage pointing at openjre6, but "java6-runtime-headless" is a
'Provides' entry that openjdk packages seem to provide for java versions
6 and 7. This should ensure we are correctly requesting *any* java 6+
implementation instead of requiring a specific java package.