prune-dns: tweak output format for easier human consumption (PR #20)

See commit messages.

I would not ordinarily bother with a PR for something so trivial, but I found I was unable to build (and therefore test) this Ruby thing without Postgres headers. It was around that point when I decided to move on to other things. :slight_smile:


I think @chipflake has unlocked the mystery of how to make rake do stuff. Merging ahead of a rebuild.