Default theme: Force HTTPS to load Google Fonts

See https://github.com/picocms/Pico/pull/346#issuecomment-213000364 (via #346 / d6accc2), thanks @smcdougall

themes/default/index.twig

@@ -10,7 +10,7 @@
         <meta name="robots" content="{{ meta.robots }}">
     {% endif %}
 
-    <link rel="stylesheet" href="//fonts.googleapis.com/css?family=Open+Sans:400,700" type="text/css" />
+    <link rel="stylesheet" href="https://fonts.googleapis.com/css?family=Open+Sans:400,700" type="text/css" />
     <link rel="stylesheet" href="{{ theme_url }}/style.css" type="text/css" />
 
     <script src="{{ theme_url }}/scripts/modernizr-2.6.1.min.js"></script>