use correct override var BUILDPACK_VENDOR_URL instead of vendor_url
use correct override var BUILDPACK_VENDOR_URL instead of vendor_url

Scalingo/python-buildpack,AltSchool/heroku-buildpack-python,heroku/heroku-buildpack-python,benmurden/heroku-buildpack-python-ember,AltSchool/heroku-buildpack-python,heroku/heroku-buildpack-python,benmurden/heroku-buildpack-python-ember,Scalingo/python-buildpack,skilljar/heroku-buildpack-python