Previously, the Ruby build system only knew how to work with gem archives,
which made it difficult to build unreleased gems from a Git repository or
released gems in tarball form.

* gnu/build/ruby-build-system.scm (gnu:unpack, gem-archive?): New procedures.
  (unpack): Use GNU build system unpack phase for non-gem sources.
  (build): Rebuild the gemspec iff the source is a gem archive.
* guix.texi ("ruby-build-system"): Mention that tarballs and directories are
  acceptable.