Cronjob via HTTP

try this ...

class Cronjob extends CI_Controller {

public function index()
{
if( $_SERVER['REMOTE_ADDR'] != $_SERVER['SERVER_ADDR'] ){
die('access is not permitted');
}
...

Try Easycron - HTTP POST Method

"In additional to conventional GET method, our cron job can be executed with HTTP POST method. In some cases, your scripts may take POST variables as input to work. We make it possible."