Attached a patch for adding Ruby 2.1.6. It works for me. This is to
try developing and supporting gems that do not necessarily work with
(latest) Ruby 2.2.2. It is also useful for the Rails people.

We ought to add Ruby 1.9 too, as well as (eventually) Rubinius (LLVM)
and JRuby (JVM).

It may appear silly to have different Ruby versions, but it is a fact
that not all gems play well on all versions and one of the strengths
of GNU Guix is that you can actually have multiple versions (similar
to what Python needs). If I want to create a gem that only works on
one specific version of Ruby, it should not matter to the end-user.

Pj.