[DRE-maint] Bug#966021: rabbit: autopkgtest fails with ruby-karmdown 2.x in experimental

[Pirate Praveen]

Package: rabbit
Version: 3.0.0-3
Severity: important

 "/usr/lib/ruby/2.7.0/rubygems/core_ext/kernel_require.rb:92:in 
`require': cannot load such file -- kramdown/parser/gfm (LoadError)\n" +

Full log
https://people.debian.org/~praveen/ruby-kramdown2-meta-build/buildlogs/jekyll.log

Seems not many breaking changes and relaxing dependency may be enough
(major one is required minimum version of ruby is 2.3 and splitting of 
some extentions as separate gems)
https://github.com/gettalong/kramdown/blob/master/doc/news/release_2_0_0.page

This version include a fix for CVE so getting this to unstable soon
would be nice.

boutil already started working on packaging ruby-kramdown-parser-gfm so 
we have to wait till that hits the archive and add build 
dependency/autopkgtest dependency on it.