Custom 1.9.3-p374 ruby

Backporting a fix for ruby bug #4266
(http://bugs.ruby-lang.org/issues/show/4266) to 1.9.3-p374.

This has the important consequence that the ruby runtime is no longer managed by
Heroku and is served out of the Do buildpacks S3 bucket.

To build a ruby distribution via vulcan including Do patches, run:
  rake 'ruby:install[1.9.3-p374]'

Upload the resulting ruby-1.9.3.tar.gz file as ruby-1.9.3-p374-do.tar.gz. The
buildpack slug install process has been modified to download from that exact
path.

Patches in the ruby-patches directory must be updated for future patchlevels.
[temp logging]