This small patch set adds a few more Ruby libraries to our arsenal, the
most important being Nokogiri: A widely used XML parsing library with
native C extensions.  The official Ruby packaging system cannot capture
non-Ruby dependencies outside of developers bundling them in their
source releases, so the ruby-nokogiri package is a great example of how
Guix improves upon language-specific package managers.  As a user of
Nokogiri, it's nice to be able to run:

    guix environment --ad-hoc ruby ruby-nokogiri -E irb

And get a Ruby REPL with Nokogiri without having to manually install
libxml2, libxslt, and the GCC toolchain from the system package manager
and hoped that things were ABI compatible when 'gem install nokogiri'
compiled the extension.

Thanks to Pjotr for writing the initial version of the Nokogiri package
and being patient for the couple of months it took me to get it reviewed
and updated for the new Ruby build system.

Patches below: