merged upstream changes

I merged with upstream changes in heroku/heroku-buildpack-python. Also, I fixed a bug where sometimes the cached copy of the package wasn't being used. Also, looks like the upstream heroku-buildpack-python changed some paths around. I updated the paths of the tarballs in npscipy-binaries to reflect that. I also added new versions of scipy, numpy and scikit-learn to npscipy-binaries.

Also: change REPO=lell to REPO=wyn in bin/steps/npscipy to clone the archives from wyn/npscipy-binaries instead of lell/npscipy-binaries

Finally: not sure why commits from 5 months ago are also included here instead of just 1 commit from merging upstream changes? maybe it's my last pull request too because I didn't also merge with your accepting my pull request 5 months ago? Anyway, I bet this will merge with no problem but let me know if there is something.