Redirect stderr to null as well

OHAI,

Installed the .dotfiles on my linux box and got a few errors... 

```
/usr/bin/which: no brew in (/usr/lib/courier-imap/bin:/usr/local/bin:/bin:/usr/bin:/usr/local/sbin:/usr/sbin:/sbin:/home/jc/bin)
/usr/bin/which: no rbenv in (/usr/lib/courier-imap/bin:/usr/local/bin:/bin:/usr/bin:/usr/local/sbin:/usr/sbin:/sbin:/home/jc/bin)
```

So I silenced them.