Fix alert container

{{cookiecutter.repo_name}}/{{cookiecutter.repo_name}}/templates/_layouts/base.html

@@ -40,14 +40,14 @@
     {% with messages = get_flashed_messages(with_categories=true) %}
     {% if messages %}
     <div class="row">
-      <div class="span12">
+      <div class="col-md-12">
         {% for category, message in messages %}
         <div class="alert alert-{{ category }}">
           <a class="close" title="Close" href="#" data-dismiss="alert">&times;</a>
          {{message}}
         </div><!-- end .alert -->
         {% endfor %}
-      </div><!-- end span -->
+      </div><!-- end col-md -->
     </div><!-- end row -->
         {% endif %}
     {% endwith %}