I'd like to refactor this slightly to clarify that this will overwrite the user's existing installation of Appledoc:

> If you'd like to build Subliminal's documentation locally, run these steps to install Appledoc 2.1:
> > Note that this is not the latest version of Appledoc (see [here](https://github.com/inkling/Subliminal/issues/71)), and will overwrite whatever you might currently have installed. You can just let Travis test that the documentation builds correctly if you prefer.