Debian packaging improvements

Some small changes to make the Riemann deb nicer to work with.
Ah, I hadn't seen those.  I was about to point out that people wanting to use the Oracle JRE from a tarball ought to make use of [make-jpkg](https://wiki.debian.org/JavaPackage), because it's quite easy and effective.  I see that already came up in #248, though.  My personal opinion is that if you're not going to install Java as a deb, you might as well install Riemann from a tarball as well. This one is up to you, of course - it's easy enough for me to keep building tweaked debs that include these java dependencies you would prefer not to include them by default.