Hello Guix hackers,

This patch represents a large change to how we're building Ruby
packages.  Previously, we would download tarballs (mostly auto-generated
ones on GitHub), build the .gem archive, then install it to the output
directory.  This method presented several issues.  No one besides us is
actually using these tarballs to build gems, and often the gems will not
build outside of a git repository due to the proliferation of 'git
ls-files' usage when building the gem.

After some more exploration, those of us interested in Ruby packaging
agreed that the pre-made .gem archives hosted on rubygems.org are in
fact source code, and that our build system could be greatly simplified
by using them.  Using these gem archives will also allow for a 'guix
import rubygems' tool to be written, greatly accelerating our packaging
efforts.

One known caveat is that not all gem archives include the test suite, so
sometimes we have to do without tests.  However, the Ruby dependency
graph already has other testing issues like rampant circular
dependencies, so I don't think this issue is a big deal in comparison.
Anyway, I only had to disable a couple of test suites for existing gems,
and I was happy that the issue wasn't too widespread among our
(currently small) sample size.

One other caveat is that downloading gems from rubygems.org currently
*does not* work because the HTTP responses include an invalid
Content-Type header.  I added a temporary hack to do my development
work (not in this patch), and I'm currently waiting on a response from
the rubygems.org admins to see if they will fix the issue.  So, this
patch should not be applied until this is resolved.

See: http://help.rubygems.org/discussions/problems/21223-invalid-content-type-header-when-downloading-gem-archive?unresolve=true

Without further ado, the patch: