Fix Flask app after upgrading to Heroku’s cedar-14 stack

After upgrading my cedar stack to cedar-14 and trying to push some new code I added to hnrss, Heroku error’d out in spectacular fashion comaining about pip vendor, a bad hashlib, and a missing urllib3.

After some searching around, I finally found a solution on Stack Overflow:

$ heroku plugins:install https://github.com/heroku/heroku-repo.git
$ heroku repo:purge_cache -a <project>

After running this, I ran git push again. It built everything from scratch and worked like a charm.