title: `onClick` handler not firing in IE11
body: <p>I'm building my first GatsbyJS project and have a simple page where content is rendered based on the active button ("currentPage") the user has clicked on. The active button is kept in the component state.</p>
  
  <p>The page works as expected in all major browsers except IE11. I'm wondering if I need a polyfill (or something) to get this to work?</p>
  
  <p>Thanks for any info!</p>
  
  <pre><code>// handler in parent component
  handlePageChange (e) {
    e.preventDefault()
    this.setState({
      currentPage: e.target.value
    })
  }
  
  // render of parent component
  render () {
    return (
      &lt;div className={styles.clientsRightsContainer}&gt;
        &lt;Buttons
          a={{ label: 'Label A', onClick: this.handlePageChange, value: 'pageA' }}
          b={{ label: 'Label B', onClick: this.handlePageChange, value: 'pageB' }}
      /&gt;
  }
  
  ...
  
  // handler gets passed down to child component Buttons via `a` and `b` props
  const Buttons = ({ a, b }) =&gt; (
    &lt;div className={styles.container}&gt;
      &lt;button onClick={a.onClick} value={a.value}&gt;{a.label}&lt;/button&gt;
      &lt;button onClick={a.onClick} value={b.value}&gt;{b.label}&lt;/button&gt;
    &lt;/div&gt;
  )
  export default Buttons
  </code></pre>