Add `auto` to the GA tracker create method

The Google Universal Analytics now supports automatic cookie domain
configuration simply by adding `auto` to the analytics tracker create
method: `ga('create', 'UA-XXXX-Y', 'auto');`.

From https://developers.google.com/analytics/devguides/collection/analyticsjs/domains#auto:

  "Automatic Cookie Domain Configuration simplifies cross domain
   tracking implementations by automatically writing cookies to the
   highest level domain possible when the auto parameter is used.
   When used on the domain www.example.co.uk, it will try to write
   cookies in the following order:

     1. co.uk
     2. example.co.uk
     3. www.example.co.uk

   Analytics.js will fail to write a cookie on co.uk but will succeed
   on example.co.uk. Since a cookie was succesfully written on a higher
   level domain, www.example.co.uk will be skipped"

So, when this parameter is omitted (as it was prior to this commit),
Google Universal Analytics will default to using `location.hostname`,
which will include the subdomain, preventing websites with multiple
subdomains from being able to automatically track unique users across
those subdomains.

Note: The `auto` parameter is now officially part of the recommended
Google Universal Analytics snippet.
https://developers.google.com/analytics/devguides/collection/analyticsjs/

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Ref: https://developers.google.com/analytics/devguides/collection/analyticsjs/domains#auto
Close: h5bp/html5-boilerplate#1562.