title: Active admin - checkbox doesn't make batch action enabled
body: <p>I install </p>
  
  <pre><code>gem 'activeadmin',         github: 'gregbell/active_admin', branch: 'rails4'
  </code></pre>
  
  <p>and it works fine but.. When i click on checkbox for some resources, batch action doesn't change to enabled. I have:</p>
  
  <pre><code>  index do |description|
     selectable_column
     column :id
     column :user
     column :description
     default_actions
    end
  
  
  
  batch_action :destroy, :confirm =&gt; "delete ??", :plural_model =&gt; "descriptions" do |selection|
  Profile.find(selection).each { |d| d.reset_description! }
  redirect_to collection_path
    end
  </code></pre>
  
  <p>and batch action for :destroy. Is active admin works correct in rails 4? </p>