Switch to using an ico file.

Works in all browsers plus it includes a 32x32 icon alongside with the 16x16 one.

site/_includes/top.html

@@ -8,7 +8,7 @@
   <link rel="alternate" type="application/atom+xml" title="Recent commits to Jekyll’s master branch" href="{{ site.repository }}/commits/master.atom">
   <link rel="stylesheet" href="//fonts.googleapis.com/css?family=Lato:100,300,400,700,900,100italic,300italic,400italic,700italic,900italic">
   <link rel="stylesheet" href="/css/screen.css">
-  <link rel="icon" type="image/png" href="/favicon.png">
+  <link rel="icon" type="image/x-icon" href="/favicon.ico">
   <!--[if lt IE 9]>
   <script src="/js/html5shiv.js"></script>
   <script src="/js/respond.min.js"></script>