use pushdminus

On my linux box `pushdminus` was set by default. This caused to work the stack the other way around.
To resolve this issue I've set `pushdminus` from oh-my-zsh. This way on all system it's the same.

It also would have been possible to `unsetopt pushdminus`. But because "go back one in the stack" is `cd -` and `-` is easier to type than a `+` (on a US keyboard) I've preferred this way.

(this is the proper pull request for #1581)