title: Activeadmin form select dropdown update
body: <p>I have a Category Model and a SubCategory Model and id like the SubCategory select input to refresh depending on the SubCategories that are associated to the specific Category I have selected.</p>
  
  <pre><code>form do |f|
      f.inputs do
          f.input :title
          f.input :category, as: :select, collection: Category.all, :input_html =&gt; { :class =&gt; 'chzn-select', :width =&gt; 'auto', "data-placeholder" =&gt; 'Click' }
          f.input :sub_category, as: :select, collection: SubCategory.all, :input_html =&gt; { :class =&gt; 'chzn-select', :width =&gt; 'auto', "data-placeholder" =&gt; 'Click' }
      end
      f.actions
  end
  </code></pre>