title: Rails: Simple form + bootstrap - When errors, fields not turning red
body: <p>I have started using Simple-form and Bootstrap and I have tried to follow this reference: <a href="https://github.com/rafaelfranca/simple_form-bootstrap/blob/master/app/views/articles/_form.html.erb" rel="noreferrer">Simple form + Bootstrap</a> but I don't know what is going on because when a field is failing, here is what happens:</p>
  
  <p><img src="https://i.stack.imgur.com/7Z4k6.png" alt="No red border in no correct field"></p>
  
  <p>Regarding this screenshot I have a question: <br>
  1) As you see, the Price field is not being red surrounded. How can I do that?
  Here is my code for the form:</p>
  
  <pre><code>&lt;%= simple_form_for @lesson, :html =&gt; { :class =&gt; 'well' } do |lesson_form| %&gt;
  &lt;% if lesson_form.error_notification %&gt;
      &lt;div class="alert alert-error fade in"&gt;
        &lt;a class="close" data-dismiss="alert" href="#"&gt;&amp;times;&lt;/a&gt;
        &lt;%= lesson_form.error_notification %&gt;
      &lt;/div&gt;
  &lt;% end %&gt;
    &lt;%= lesson_form.input :title %&gt;
    &lt;%= lesson_form.input :category %&gt;
    &lt;%= lesson_form.input :description %&gt;
    &lt;%= lesson_form.input :price %&gt;
    &lt;%= lesson_form.button :submit, :label =&gt; 'Create', :class =&gt; 'btn btn-primary btn-large' %&gt;
  &lt;% end -%&gt;
  </code></pre>