The webcall hooks continue to exist in the new deployment fortrabbit.yml file. Simply use the post-deploy hook and set an url, like so:
post-deploy:
# alternatively/additionally an URL can be specified which is called after
# deployment
url: http://domain.tld/callback
# token which will be send as request header `X-Frbit-Token` to verify that
# the request originated from this post-deploy call (defaults to "unset")
token: your-own-token
In your callback, you receive the commit ID: http://fortrabbit.com/docs/in-depth/git-hooks/webcall#the-callback-url