Moving application `javascript_include_tag` to the HTML <head>.

Rails 4 now includes Turbolinks by default, therefore adding JavaScript
at the bottom of the <body> element will not play nice with Turbolinks.
The application's JavaScript should be loaded in the HTML <head>.

If a user wants to disable turbolinks, and move the application's
JavaScript files back to the bottom of the HTML <body> element, then
they can do so, but the default templates that gets generated by the
"foundation:install" generator should declare the JavaScript in the
HTML <head>.

Sources:
* https://github.com/rails/turbolinks#evaluating-script-tags
* http://stackoverflow.com/a/20169679/631834