heroku/heroku-buildpack-ruby — pull request #276 by tt (2014-06-16 21:21:48+00:00)
Title: Support user overwrites

I can vendor my own set of binaries by setting `BUILDPACK_VENDOR_URL`. Unfortunately, setting this on Heroku has no effect as the user environment doesn't make it into `ENV`. The method `env` accommodates for this, so the change allow overwrites.