Update the plugin to handle JSON-encoding and SSL requests in WordPress 3.7

WordPress 3.7 changes the way plugin updates are sent to the plugin update API. WP-LESS is currently breaking all updates and generating notices due to the `http_requests_args()` method which needs updating.

More info: http://make.wordpress.org/core/2013/10/25/json-encoding-ssl-api-wordpress-3-7/