Don't assume the app is in /app, expose env variables to subsequent buildpack

[Photonios]

• Don't assume the root is going to be /app. There are some buildpacks out there that make other buildpacks run in subdirectories and this assumption breaks them.

• Expose environment variables to subsequent buildpacks (such as the Python one) so that they can also read them.

This seems to fix: #37 #39 #31

It might be a good idea to also pull in: https://github.com/dschep/heroku-geo-buildpack/commit/5ae45e57353aab6a9fb2f0443e9ea0974d8b29ba

To fix support for heroku-16.

[mobilutz]

Is there any information on this topic? I would love to use this functionality, but there is merge conflict problem 🙁

[Photonios]

Hey @mobilutz, I think I managed to resolve the conflict 😄