In #158 the event and context arguments to the callback got combined into one.
The documentation was still referring to the event object, which is no longer present.
These instructions referred to a file `autoresponder.js` which does not exist (a) for a plugin in general, or (b) for the autoresponder plugin in particular. The plugin template repo contains a file named `index.js` to export the plugin module, and this naming scheme seems to have been adopted widely (e.g. autoresponder, stale plugins both use `index.js`).