Update app.json metadata (#243)

* Remove the `image` field, since the `heroku.yml` languages feature was
   sunset some time ago, and the language docker images no longer exist.
* Add the `website` field so the Heroku Button deploy form links to the
   Getting started guide.

See:
https://devcenter.heroku.com/articles/heroku-button
https://devcenter.heroku.com/articles/app-json-schema

Author: edmorley

app.json

@@ -1,8 +1,8 @@
 {
   "name": "Start on Heroku: Python",
   "description": "A barebones Python app, which can easily be deployed to Heroku.",
-  "image": "heroku/python",
   "repository": "https://github.com/heroku/python-getting-started",
+  "website": "https://devcenter.heroku.com/articles/getting-started-with-python",
   "keywords": ["python", "django"],
   "env": {
     "DJANGO_SECRET_KEY": {