title: Yii2 required validation rule on "select2" widget
body: <p>I'm using <a href="http://demos.krajee.com/widget-details/select2" rel="nofollow noreferrer">kartik select2 widget</a> in Yii2 framework. Required validation rule doesn't work on it. <br>
  Here is my view code:</p>
  
  <pre><code>$form-&gt;field($model, 'city')-&gt;widget(\kartik\select2\Select2::classname(), [
              'data' =&gt; $cities,
              'options' =&gt; [
                  'class' =&gt; 'form-control',
                  'placeholder' =&gt; 'Please select city...',
                  'multiple' =&gt; false,
              ],
              'pluginOptions' =&gt; [
                  'allowClear' =&gt; true
              ],
          ])-&gt;label('City');
  </code></pre>
  
  <p>Here is my model rule code:</p>
  
  <pre><code>[['city'], 'required'],
  [['city'], 'integer']
  </code></pre>
  
  <p>Any idea to make the dropdown required?</p>