Migrate from analytics.js to gtag.js (jekyll#281)

* Replace analytics.js with gtag.js

_includes/google-analytics.html

@@ -1,12 +1,9 @@
+<script async src="https://www.googletagmanager.com/gtag/js?id={{ site.google_analytics }}"></script>
 <script>
-if(!(window.doNotTrack === "1" || navigator.doNotTrack === "1" || navigator.doNotTrack === "yes" || navigator.msDoNotTrack === "1")) {
-  (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-disable-{{ site.google_analytics }}'] = window.doNotTrack === "1" || navigator.doNotTrack === "1" || navigator.doNotTrack === "yes" || navigator.msDoNotTrack === "1";
+  window.dataLayer = window.dataLayer || [];
+  function gtag(){dataLayer.push(arguments);}
+  gtag('js', new Date());
 
-  ga('create', '{{ site.google_analytics }}', 'auto');
-  ga('send', 'pageview');
-}
+  gtag('config', '{{ site.google_analytics }}');
 </script>
-