Change reference from deprecated wpApiOptions to WP_API_Settings.root

Based on a change to the JSON REST API plugin (https://github.com/WP-API/WP-API/pull/321),
we now need to reference WP_API_Settings.root which now is responsible for rendering the home directory and the wpjson subdirectory. Otherwise, you get a 'wpApiOptions is not defined' error, which is currently happening on the theme's demo site on the single post view:
http://demo.wedevs.com/backbone/#/posts/1/hello-world/.