Fix duplicate GTM ID value in head.html

[kamui]

Reported in issue #11.

GTM docs specify that the snippet should look like this:

<!-- Google Tag Manager -->
<script>(function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':
new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],
j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src=
'https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);
})(window,document,'script','dataLayer','GTM-XXXX');</script>
<!-- End Google Tag Manager -->

The problem in head.html is that it is adding the GTM twice. The dataLayer method is how it's suggested in the snippet above, but head.html also interpolates it into the gtm.js string itself.