title: select2 display not updating label text but value is changing
body: <p>I am using <strong>select2 v4.0.3</strong> along with JQuery.
  I initialize my selectors as:</p>
  
  <pre><code>$("#datatype").select2({
          theme: "classic"
          }); 
  
   $("#unit").select2({
          theme: "classic"
          }); 
  </code></pre>
  
  <p>Then I want to detect a change on <code>#datatype</code> and change the value of <code>#unit</code> which I do as follows: </p>
  
  <pre><code>$("#datatype").on('select2:select',function () {
      $("#unit").val('21'); //udpate unit type to be 'none'
      });
  </code></pre>
  
  <p>The problem is that the value of <code>#unit</code> is getting set to 21 but the displayed label for that selector is not updating. Any suggestions ?</p>
  
  <p>It seems as if select2 is not aware of the event or that it has not been setup properly to listen to changes. I am not sure what the solution is. I am new to web technologies.</p>