[DRE-maint] Bug#903285: bundler FTBFS: fails to find ronn

[Helmut Grohne]

Source: bundler
Version: 1.16.1-2
Severity: serious
Tags: ftbfs

bundler fails to build from source since ronn got split out from
ruby-ronn into a separate package ronn. Fixing this problem will involve
adding ronn to Build-Depends as the build system tries to invoke ronn
with "ruby2.5 -S ronn". Adding a transitional dependency was not
possible as it would have created a dependency cycle.

I'm less sure about whether the ruby-ronn dependency could be dropped,
because ronn is mentioned in a .gemspec file. I don't quite understand
how bundler uses ronn, but it might be that it uses both the script
(ronn) and the ruby module (ruby-ronn). If that is the case, it actually
needs to depend on both.

Helmut