Use new Google snippet

This should work with both old Universal Analytics (UA) and new Google Analytics v4 (GA4).

Resolves pages-themes#63

_includes/head-custom-google-analytics.html

@@ -1,10 +1,10 @@
 {% if site.google_analytics %}
-  <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','//www.google-analytics.com/analytics.js','ga');
-    ga('create', '{{ site.google_analytics }}', 'auto');
-    ga('send', 'pageview');
-  </script>
+<script async src="https://www.googletagmanager.com/gtag/js?id={{ site.google_analytics }}"></script>
+<script>
+  window.dataLayer = window.dataLayer || [];
+  function gtag(){dataLayer.push(arguments);}
+  gtag('js', new Date());
+
+  gtag('config', '{{ site.google_analytics }}');
+</script>
 {% endif %}