Set `merged' action if `merged' key is true
Set `merged' action if `merged' key is true

The GitHub API doesn't actually return `action = merged' when a PR
is merged; it just sets `pull_request.merged = true'.

From the Description field for the Action key:
https://developer.github.com/v3/activity/events/types/#pullrequestevent

debugger22/github-audio,debugger22/github-audio