Django/Heroku: How to load settings file from Procfile

Django/Heroku: How to load settings file from Procfile

I'm trying to set my Procfile to load a specific settings file on Heroku:
web: newrelic-admin run-program python manage.py run_gunicorn -b
0.0.0.0:\$PORT
-w 9 -k gevent --max-requests 250 --preload --settings=setting_files.site1
However the standard settings.py loads instead. Any idea why this might be
happening?