In good tradition, ruby made a new release today (25/12)[0].

I tried building some packages with the new version, but ruby-minitest
complains that Rake 12 is too new (even with the latest minitest). There
have been some core changes as well, with Fixnum and Bignum now merged
into a single Integer class.

I suggest that we keep ruby 2.3 as the main "ruby" variable until the
ecosystem catches up. Users will still get the latest version when
using `guix package` or `guix environment`. WDYT?

Patch attached. Not sure about the commit message.

0: https://www.ruby-lang.org/en/news/2016/12/25/ruby-2-4-0-released/