Fix google analytics script

- This was fixed in: jekyll/minima#825; but 'github-pages' gem uses 2.5.1

Author: rsn491

_includes/google-analytics.html

@@ -0,0 +1,8 @@
+<script async src="https://www.googletagmanager.com/gtag/js?id={{ site.google_analytics }}"></script>
+<script>
+  window.dataLayer = window.dataLayer || [];
+  function gtag(){window.dataLayer.push(arguments);}
+  gtag('js', new Date());
+
+  gtag('config', '{{ site.google_analytics }}');
+</script>