`SLAlertHandler` now optionally logs alerts as they are handled.

Allowing tests to observe and debug alert handling,
especially the automatic handling of unexpected alerts.
Force Travis to use appledoc 2.1. (fixes #70, refs #71)

By installing using the last version of the formula to reference 2.1,
as found using `brew versions appledoc`.
Merge pull request #69 from inkling/jeff/alert_handler_logging

`SLAlertHandler` now optionally logs alerts as they are handled.