app/views/shared/_google_analytics.html.erb
<% if Rails.env.production? && ENV["GOOGLE_ANALYTICS_APP_ID"].present? -%>
<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', '<%= ENV["GOOGLE_ANALYTICS_APP_ID"] -%>', '<%= ENV["GOOGLE_ANALYTICS_HOSTNAME"] -%>');
ga('send', 'pageview');
</script>
<% end -%>