Attached a Ruby module in preparation of Nokogiri support.

(I am pretty close as described in
https://github.com/pjotrp/guix-notes/blob/master/RUBYGEMS-Nokogiri.org).

Cucumber support comes later (it depends on Nokogiri itself).

David: there are quite a few modules now that build gems through 'rake
gem'.  Would it be an idea to try that by default when there is no
gemspec file?

Pj.