Add automatic migrations to heroku deploys (#1951)

heroku now has a new feature for running tasks as part of deployment. Perfect for automatic migrations.
https://devcenter.heroku.com/articles/release-phase#specifying-release-phase-tasks

docs/deployment-on-heroku.rst

@@ -47,7 +47,6 @@ Run these commands to deploy the project to Heroku:
 
     git push heroku master
 
-    heroku run python manage.py migrate
     heroku run python manage.py createsuperuser
     heroku run python manage.py collectstatic --no-input
 

{{cookiecutter.project_slug}}/Procfile

@@ -1,3 +1,4 @@
+release: python manage.py migrate
 web: gunicorn config.wsgi:application
 {% if cookiecutter.use_celery == "y" -%}
 worker: celery worker --app={{cookiecutter.project_slug}}.taskapp --loglevel=info