brek out heroku specific requirements and install via Procfile release action

Procfile

@@ -1,2 +1,3 @@
+release: pip install -r requirements_heroku.txt
 release: yes "yes" | python manage.py migrate
 web: gunicorn bakerydemo.heroku_wsgi --log-file -

requirements.txt

@@ -3,9 +3,3 @@ elasticsearch
 wagtail
 wagtailfontawesome
 Pillow
-
-# Dependencies for Heroku deployment
-dj-database-url==0.4.1
-whitenoise==3.2.2
-gunicorn==19.6.0
-psycopg2==2.6.2

requirements_heroku.txt

@@ -0,0 +1,5 @@
+# Additional dependencies for Heroku deployment
+dj-database-url
+whitenoise
+gunicorn
+psycopg2