app/views/application/_flash.html.erb
<% flash.each do |key, value| %>
<div class="container flash-message-row animated slide-in-slide-out">
<div class="flash-message">
<%= content_tag :div, class: "alert alert-dismissible #{flash_key_to_bootstrap_class(key)}", role: 'alert' do %>
<button type="button" class="close" data-dismiss="alert" aria-label="<%= t('close') %>">
<span aria-hidden="true">×</span>
</button>
<%= value %>
<% end %>
</div>
</div>
<% end %>