Co-Authored-By: Pjotr Prins <pjotr.public01@thebird.nl>

* guix/build-system/ruby.scm (lower): Remove git dependency.
  (rubygems-uri): New procedure.
* guix/build/ruby-build-system (gitify): Delete.
  (unpack): Use 'gem unpack' utility.
  (check): Add docstring.
  (build): Repack modified gem.
  (install): Rebuild unpacked gem and install it.
  (%standard-phases): Remove gitify and build phases.
* gnu/packages/ruby.scm (ruby-hoe, ruby-rake-compiler, ruby-i18n,
  ruby-rspec-support, ruby-rspec-core, ruby-diff-lcs-for-rspec,
  ruby-rspec-expectations, ruby-rspec-mocks, ruby-rspec, bundler,
  ruby-useragent, ruby-bacon, ruby-arel, ruby-connection-pool,
  ruby-net-http-persistent, ruby-minitest, ruby-minitest-sprint,
  ruby-minitest-bacon, ruby-daemons, ruby-git, ruby-slop,
  ruby-multipart-post): Convert to new build system.
* doc/guix.texi (ruby-build-system): Document the gem archive requirement.