sage-cleaner: fix rm_rf on OS X
Trac #16293: Fix sage-cleaner's rm_rf function on OS X

On OS X, if you use `os.unlink` on a directory, you get an OSError,
"[Errno 1] Operation not permitted". We need to catch this so that
`sage-cleaner` works properly. (On linux machines, you seem to get
OSError, "[Errno 21] Is a directory".)

URL: http://trac.sagemath.org/16293
Reported by: jhpalmieri
Ticket author(s): John Palmieri
Reviewer(s): Leif Leonhardy
Updated Sage version to 6.2