Updated Google Analytics code (alshedivat#123)

GA site tag was updated today, misc.html has been changed to reflect the same.

Author: kakodkar

_includes/scripts/misc.html

@@ -6,15 +6,14 @@
 {% endif %}
 
 {% if site.enable_google_analytics %}
-<!-- Google Analytics -->
+<!-- Global site tag (gtag.js) - Google Analytics -->
+<script async src="https://www.googletagmanager.com/gtag/js?id={{ site.google_analytics }}"></script>
 <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');
+  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>
 {% endif %}