cleanup of the heroku configuration

this removes the general `postinstall` call to `bin/heroku` and instead
puts it into a heroku-prebuild hook. At the same time, env vars get
updated to use the `CMD` prefix. The configured buildpacks were not used.
Finally, npm run build is now automatically
done by Heroku.

Signed-off-by: Claudius <opensource@amenthes.de>

package.json

@@ -10,8 +10,8 @@
     "jsonlint": "find . -not -path './node_modules/*' -type f -name '*.json' -o -type f -name '*.json.example' | while read json; do echo $json ; jq . $json; done",
     "standard": "echo 'standard is no longer being used, use `npm run eslint` instead!' && exit 1",
     "dev": "webpack --config webpack.dev.js --progress --colors --watch",
+    "heroku-prebuild": "bin/heroku",
     "build": "webpack --config webpack.prod.js --progress --colors --bail",
-    "postinstall": "bin/heroku",
     "start": "sequelize db:migrate && node app.js",
     "doctoc": "doctoc --title='# Table of Contents' README.md"
   },