_flash_messages.html.erb

<% flash.each do |type, message| %>
  <div class="alert <%= alert_class_for(type) %> alert-dismissible fade in">
    <button type="button" class="close" data-dismiss="alert"><span aria-hidden="true">&times;</span><span class="sr-only">Close</span></button>
    <%= message %>
  </div>
<% end %>

application.html.erb

<%= render partial: "shared/flash_messages", flash: flash %> 

application_helper.rb

module ApplicationHelper

  def alert_class_for(flash_type)
    {
      :success => 'alert-success',
      :error => 'alert-danger',
      :alert => 'alert-warning',
      :notice => 'alert-info'
    }[flash_type.to_sym] || flash_type.to_s
  end

end