use pushdminus

On my linux box `pushdminus` was set by default. This caused to work the stack the other way arond.
To resolve this issue I've set `pushdminus` from oh-my-zsh. This way on all system its the same.

It also would be possible to `unsetopt pushdminus`. I because an `-` is easier to type than a `+` I've preferred this way.