Fix URL for menus

Reverts 0d9656d939 (diff-5c3a1b4da16de002912dd90a78641f32R12)

Fixes #229

layouts/partials/site-navigation.html

         <ul class="pl0 mr3">
           {{ range .Site.Menus.main }}
           <li class="list f5 f4-ns fw4 dib pr3">
-            <a class="hover-white no-underline white-90" href="{{ .Permalink }}" title="{{ .Name }} page">
+            <a class="hover-white no-underline white-90" href="{{ .URL }}" title="{{ .Name }} page">
               {{ .Name }}
             </a>
           </li>