Change app.json to use Fission's Heroku addon

[MCTaylor17]

The app.json file currently references env variables for setup. This can be simplified/streamlined by adding Fission's Heroku addon like so:

{
  "addons": [
    "jawsdb",
    "mailgun",
    {
      "plan": "interplanetary-fission:test"
    }
  ],
}