Update app.json

Author: Edward Muller

app.json

@@ -3,8 +3,6 @@
   "description": "Let people invite themselves to your Slack",
   "logo": "https://i.imgur.com/8mRVeMn.png",
   "keywords": ["go", "slack"],
-  "image": "heroku/go:1.6",
-  "mount_dir": "http://github.com/flexd/slackinviter",
   "website": "https://github.com/flexd/slackinviter",
   "repository": "https://github.com/flexd/slackinviter",
   "env": {