[DRE-maint] Bug#796497: ruby-ronn: depends on obsolete ruby-hpricot

[Sebastian Boehm]

Just for the record: ronn's upstream seems to be dead. The latest
release is more than five years old and there haven't been any
upstream commits for over two years.

While ronn is a useful tool, asciidoc and asciidoctor are viable
alternatives and are actively maintained as well as available in
stretch and jessie.