navbar: reverse-sort pages

This puts the 2015 pages first. We may want to just do this
manually, though.

Author: peff

_includes/navbar.html

@@ -2,7 +2,7 @@
   <ul>
     <li><a href="/">Home</a>
     <li><a href="about.html">About</a>
-    {% for node in site.pages %}
+    {% for node in site.pages reversed %}
       {% if node.navbar != false %}
       <li><a href="{{node.url}}">{{node.title}}</a>
       {% endif %}