Fix pages variable in navigation bar

Fixes issue duilio#91, pages do not show in the nav. bar.

templates/_includes/navigation.html

@@ -31,7 +31,7 @@
     <li><a href="{{ link }}">{{ title }}</a></li>
   {% endfor %}
   {% if DISPLAY_PAGES_ON_MENU %}
-    {% for page in PAGES %}
+    {% for page in pages %}
       <li><a href="{{ SITEURL }}/{{ page.url }}">{{ page.title }}</a></li>
     {% endfor %}
   {% endif %}