Updates application layout template with JavaScript at bottom of page.

Prevailing wisdom in front end performance states that CSS should be
loaded at the top of a page and JavaScript at the bottom. This updates
the generated application layout to follow that convention.

The application layout will now look like this by default:

    <!DOCTYPE html>
    <html>
    <head>
      <title>ApplicationName</title>
      <%= stylesheet_link_tag 'application', media: 'all',
            'data-turbolinks-track' => true %>
      <%= csrf_meta_tags %>
    </head>
    <body>

    <%= yield %>

    <%= javascript_include_tag 'application',
          'data-turbolinks-track' => true %>
    </body>
    </html>