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.
Merge pull request #1006 from jordansissel/fix-pull-893

Ubuntu/Debian should depend on 'java-runtime-headless'
Depend on java6 at minimum.
Always be UTC (bugfix)
Merge pull request #1010 from jordansissel/fix-date-tz-bug

Always be UTC (bugfix)
Merge pull request #1011 from jordansissel/deb-java6

Depend on java6 at minimum.