Fix sanityinc/add-subdirs-to-load-path to only add subdirs

I noticed that `~/.emacs.d/site-lisp/README` was in my load-path, this should fix it so `sanityinc/add-subdirs-to-load-path` only adds subdirs of `~/.emacs.d/site-lisp` instead of all non-hidden files and subdirs.

I tested the fix on Linux using GNU Emacs built from the official git sources on 2014-11-04.