Update _yaourt

I have the next function in `~/.zshrc`:

    pacman () {
      /usr/bin/sudo /usr/bin/pacman $* && echo "$*" | grep -q "S\|R" && rehash
    }

And I have the next line in `/etc/yaourtrc`:

    PACMAN="/usr/bin/pacman"

But if I try to get autocomplite for `yaourt -Ss` (for example) it requires a sudo password. With this patch path to `pacman` (`PACMAN` variable) will be read from the configuration file and this command will not require the password.

It is a correct variant :)