Support pulling a tarball defined in env.

I have a case where I need to pull some files from S3 during a Heroku deploy, but the tarball URL can't be defined statically in the repo (because I have both staging and production deployments on Heroku, and the two deployments need to pull a different set of files). So I added a feature to allow specifying the S3 tarball URL via a Heroku config var S3_BUILDPACK_TARBALL. I didn't allow for multiple tarball URLs specified in this config var as that would be a bit more complex and I didn't need it.