Bundler 2

[Josh Holland]

Hi Guix,

In the course of some fairly involved yak-shaving, I found the need to
run a Ruby package, and tried running `guix environment --ad-hoc bundler
ruby`.  However, running it produced an error:

$ bundle install
Traceback (most recent call last):
        2: from 
/gnu/store/5flpa8h2adwb86n49r7p3mbp11p0jafr-profile/bin/bundle:23:in `<main>'
        1: from 
/gnu/store/9yqh0g1p5bmxar8dlfp84j4py3j631jv-ruby-2.6.5/lib/ruby/2.6.0/rubygems.rb:302:in
 `activate_bin_path'
/gnu/store/9yqh0g1p5bmxar8dlfp84j4py3j631jv-ruby-2.6.5/lib/ruby/2.6.0/rubygems.rb:283:in
 `find_spec_for_exe': Could not find 'bundler' (2.1.0.pre.2) required by your 
/home/josh/c/gnome-dbus-emulation-wlr/Gemfile.lock. (Gem::GemNotFoundException)
To update to the latest version installed on your system, run `bundle update 
--bundler`.
To install the missing version, run `gem install bundler:2.1.0.pre.2`

Indeed I see that gnu/packages/ruby.scm only packages Bundler v1.17.3,
though Bundler 2 was released in January 2019 [0].  I'm not hugely
familiar with the Ruby ecosystem, but reading the release announcement
suggests that they expect users/developers to have both Bundler 1 and
Bundler 2 available.  If I hadn't seen this, I'd have gone ahead and
submitted a patch simply upgrading the Bundler package definition, but
I'm not sure if that will then immediately break every package depending
on it that expects Bundler 1.

Is this a valid concern?  Should Guix provide packages for both Bundler
1 and Bundler 2, or would just Bundler 2 be sufficient?  I'll still
write a definition for the new version, but I'll hold off submitting a
patch until I've had some feedback.

[0]: https://bundler.io/blog/2019/01/03/announcing-bundler-2.html

Thanks,
--
Josh Holland