Create a new Bot Integration under services/new/bot.
On the next screen, note the API token.
Set SLACK_API_TOKEN from the Bot integration settings on Slack.
heroku config:add SLACK_API_TOKEN=...
A required API key from https://console.developers.google.com/apis. You must create a new API project for Google Custom Search and add credentials.
Google Custom Search ID from https://cse.google.com/cse. When set the custom search engine will be used instead of Google Web Search.
SlackGoogleBot replies with animated GIFs. While it's currently not necessary, you may need to set GIPHY_API_KEY in the future, see github.com/Giphy/GiphyAPI for details.
Heroku free tier applications will idle. Either pay 7$ a month for the hobby dyno or use UptimeRobot or similar to prevent your instance from sleeping or pay for a production dyno.