Drop __autoload.

spl_autoload_register exists in >=5.1.2, we require >=5.3.2
We no longer support hphpc and it looks like hhvm supports spl_autoload_register

We've got no reason to keep using __autoload.
We'll also need to exclusively use spl_autoload_register when we start using composer.json stuff.

Change-Id: I694b7b96825e5d136fa76461511efc9a002149fa