title: html2canvas throws a syntax error using promise functions in ie11
body: <p>Hello I am having a strange issue when running my site on ie11 when trying to use html2canvas where it throws a generic 'synax error' when the .then function is called. i have attempted to remedy this by using polyfill.min.js (from <a href="https://github.com/taylorhakes/promise-polyfill" rel="nofollow noreferrer">this link</a>, i am using the CDN for simplicity's sake), but promises seem to still not be working. i was following instructions from RedStapler the youtube help channel (<a href="https://www.youtube.com/watch?v=tKDrWSW7XYA" rel="nofollow noreferrer">video</a>).</p>
  
  <p>does anyone know why this still isn't working, and have a solution? thanks!</p>
  
  <pre><code>html2canvas(document.getElementById("print"), {
      height: 990,
      width: 1760,
      logging:false
  }).then(canvas =&gt; { //syntax error is thrown on this line
      document.getElementById('png-container').appendChild(canvas);
      $('#png-container &gt; canvas').attr('id','thecanvas');  
      title = $('#title').text(); 
      exportCanvasAsPNG('thecanvas',title);
  });
  </code></pre>