Use the free Deploy Hook addon from Heroku. Run this command to add it to your Heroku app:
heroku addons:create deployhooks:http --url="https://api.rollbar.com/api/1/deploy/?access_token=POST_SERVER_ITEM_ACCESS_TOKEN&environment=production"
Note that this includes your Rollbar project access token and the environment name "production".
If you already have an HTTP deploy hook set up, you'll need to remove it first:
heroku addons:destroy deployhooks:http