Added support for Google Analytics async snippet loading (#921)

Snippet from Google: https://developers.google.com/analytics/devguides/collection/analyticsjs/#alternative_async_tracking_snippet

Recommended by HTML5Boilerplate:
https://github.com/h5bp/html5-boilerplate/blob/master/dist/index.html

src/partials/integrations/analytics.html

@@ -22,13 +22,7 @@
 
 <!-- Google Analytics integration -->
 <script>
-  (function(i,s,o,g,r,a,m){
-    i["GoogleAnalyticsObject"]=r;i[r]=i[r]||function(){(i[r].q=i[r].q||
-    []).push(arguments)},i[r].l=1*new Date();a=s.createElement(o),
-    m=s.getElementsByTagName(o)[0];a.async=1;a.src=g;
-    m.parentNode.insertBefore(a,m)
-  })(window, document,
-    "script", "https://www.google-analytics.com/analytics.js", "ga");
+  window.ga=window.ga||function(){(ga.q=ga.q||[]).push(arguments)};ga.l=+new Date;
 
   /* General initialization */
   ga("create",
@@ -59,3 +53,4 @@
     });
   }
 </script>
+<script async src='https://www.google-analytics.com/analytics.js'></script>