Fixes for latest heroku python buildpack

The current scripts don't work very well with the latest buildpack from heroku (160497cb):

* `.heroku/vendor/` doesn't exist, I've changed it to use `.heroku/node/` (and create it if it doesn't exist), but I'm not sure if this is the 'correct' fix.
* errors in the script don't stop it from running

I've also noticed it's *really hard* to get post_compile script updates to 'stick' - `bin/` gets cached by heroku so replacement files don't actually ever update. This is the workaround I've found:

    heroku config:set CLEAN_VIRTUALENV=yes
    git push heroku master  # this time will nuke old post_compile scripts, but then doesn't run them
    heroku config:unset CLEAN_VIRTUALENV
    git push heroku master  # this time post_compile scripts will run

Any insights?