title: extend .list-unstyled with Bootstrap 4 is failing
body: <p>I install Bootstrap 4.0.0.0 alpha6 on Rails 5.1 with Yarn.
  I post <a href="https://stackoverflow.com/questions/43857468/install-with-yarn-bootstrap4-0-0-alpha-6-on-rails-5-1">here</a> how I installed.
  So In my .SCSS I'm trying to do some unestyled list with bootstrap 4.</p>
  
  <pre><code> ul.actions {
      @extend .list-unstyled;
      position: absolute;
      bottom: -2px;
      right: 2px;
      max-width: 45%;
      text-align: right;
   }
  </code></pre>
  
  <p>And I having an error:</p>
  
  <pre><code>"ul.actions" failed to @extend ".list-unstyled".
  The selector ".list-unstyled" was not found.
  Use "@extend .list-unstyled !optional" if the extend should be able to fail.
  </code></pre>
  
  <p>So if I put on my <code>index.html.slim</code></p>
  
  <pre><code>ul.actions.list-unstyle
  </code></pre>
  
  <p>The unstyled list works. What happened here? Was because it wasn't installed with bootstrap-sass gem? I try with <code>@include list-unstyled;</code> and nothing. How can I use this in my case?</p>