change favicon to match github

Author: Sophie Shepherd

_includes/head.html

@@ -3,7 +3,7 @@
   <meta name="viewport" content="width=device-width, initial-scale=1">
   <link href="https://fonts.googleapis.com/css?family=Roboto:300,300i,400,400i,700,700i" rel="stylesheet">
   <link href="/assets/css/index.css" rel="stylesheet">
-  <link rel="icon" type="image/png" href="{{ "/assets/favicon-128.png" | prepend: site.baseurl }}" sizes="128x128" />
+  <link rel="icon" type="image/x-icon" href="https://assets-cdn.github.com/favicon.ico">
   <script src="https://ajax.googleapis.com/ajax/libs/jquery/2.2.4/jquery.min.js"></script>
   <script src="{{ "/js/script.js" | prepend: site.baseurl }}"></script>
   <script>