How to add babel transformers?

I'm trying:

``` js
elixir.ready(() => {
    elixir.config.js.webpack.babel = {
        presets: ['es2015'],
        plugins: ['transform-class-properties'],
    }
});
elixir(mix => {
    mix.webpack('app.js', 'public/assets/app/js/app.js', 'resources/assets/js');
});
```

But it seems not to be working — class properties still fail
