title: How to use babel presets in react-rails
body: <p>I have a rails app using the react-rails gem, and I'd like to use arrow functions and static function declaration:</p>
  
  <pre><code>class Component {
    static propTypes = { ... }
  
    onClick = (e) =&gt; { ... }
  }
  </code></pre>
  
  <p>I tried adding an initializer but I don't know how to add the <code>stage-3</code> preset from babel:</p>
  
  <pre><code>// stage-3 not found
  config.react.jsx_transform_options = {
    optional: ["stage-3"]
  }
  
  // preset option doesn't exist
  config.react.jsx_transform_options = {
    preset: ["stage-3"]
  }
  </code></pre>
  
  <p>How can I use babel presets with react-rails?</p>