Use the new Google Analytics tracking code (Universal Analytics)

Google recently deployed their new tracking code as a public beta, and [recommend new installations use this](https://developers.google.com/analytics/devguides/collection/analyticsjs/):

> The analytics.js snippet is part of Universal Analytics, which is currently in public beta. New users should use analytics.js. […] To begin tracking a website using analytics.js, paste the following JavaScript snippet into your website template page so that it appears before the closing </head> tag.

I've updated `lib/scripts.php` to
1. Use this new script, and
2. Output to `wp_head()` instead of `wp_footer()` as suggested by Google.

I should state up front that I only have basic experience with the new tracking code. I just figured that Roots developers might benefit from the added flexibility, and that Roots itself should be up to date with any vendored plugins/scripts.

What's your opinion?