title: Yii2 - Upgrade Bootstrap 3 to 4
body: <p>i'm trying upgrade bootstrap from version 3 to 4, with <a href="https://github.com/yiisoft/yii2-bootstrap4" rel="nofollow noreferrer">this</a> official extension and now works partial. for example i use in my project  a widgets from 2amigos and i think is dependent from bootstrap 3. in the pages where i have a widget from 2 amigos the bootstrap3 is called always and deconfigured my layout (constructed in BS4) it's possible remove completely bootstrap 3 or other solution for not call BS3 anymore?</p>
  
  <p>other thing i see is the module Gii also layout desconfugured. </p>
  
  <p>installation</p>
  
  <pre><code>php composer.phar require --prefer-dist yiisoft/yii2-bootstrap4
  </code></pre>
  
  <p>class AppAsset</p>
  
  <pre><code>public $depends = [
          'yii\web\YiiAsset',
          'yii\bootstrap4\BootstrapAsset',
      ];
  </code></pre>
  
  <p>and i rename all from <code>yii\bootstrap</code> to <code>yii\bootstrap4</code></p>
  
  <p>i also try remove <code>yiisoft/yii2-bootstrap": "~2.0.0"</code> from <code>composer.json</code> but not work, the results is same. </p>