async load analytics javascript script tag

analytics.js

(function(){
  window['GoogleAnalyticsObject'] = 'ga';
  window['ga'] = 
    window['ga'] 
    || 
    function(){
      (window['ga'].q = window['ga'].q || [])
      .push(arguments)
    }, 
    window['ga'].l = 1 * new Date();
    
    a = document.createElement('script'),
    m = document.getElementsByTagName('script')[0];
    
    a.async = 1;
    a.src = '//www.google-analytics.com/analytics.js';
    m.parentNode.insertBefore(a, m)
})();

ga('create', 'UA-XXXXXXXX-1', 'auto');
ga('send', 'pageview');