Fix Google Analytics script

[ashmaroli]

Context

Adapted from patch #824 for 2.5-stable branch by @iTiamo

Reference

#816 (comment) :

creating a file called google-analytics in my local directory _includes with the snippet that was provided to me...

<!-- Google tag (gtag.js) -->
<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>

This caused data to start flowing into my Google Analytics instance.