jjb: accept change notifications from GitHub

jjb: accept change notifications from GitHub

In commit fac5ec4127ac1178b40ce0bd8bd176af355aadce I slowed down the Git
SCM polling for the JJB management job, with the reasoning that the
GitHub webhook would be a more responsive and efficient method of
triggering builds.

However, this was not entirely sufficient because the JJB job itself
must also be explicitly configured to accept the GitHub webhook
notifications.

Add "github" to the list of triggers for the JJB job. (This setting
corresponds to the "Build when a change is pushed to GitHub" checkbox in
Jenkins web UI.)