heroku/heroku-buildpack-ruby — pull request #115 by maletor (2013-06-28 17:12:23+00:00)
Title: Incompatible with latest heroku ruby buildpack

https://github.com/heroku/heroku-buildpack-ruby/compare/ndbroadbent:master...heroku:master#L13R65

Possibly related to the way cache has been abstracted.