Fatal error: Class 'Phalcon\DI\FactoryDefault' not found in

Hi,

I followed the installation instructions here:

https://docs.phalcon.io/en/latest/reference/install.html

I'm on CentOS 6.6, PHP 5.3, latest PhalconPHP build

When I navigate to my website from the browser I'm getting this error message:

Fatal error: Class 'Phalcon\DI\FactoryDefault' not found in /home/web/sites/curbquote-phalcon/public/index.php on line 25

Tried different version of PHP (5.4) but still no joy

I also tried to downgrade PhalconPHP to 1.2.6 which I believe was used when we originally deployed our application, but still nothing.

Here's the code in index.php

<?php
/** set default time zone **/
date_default_timezone_set('America/Boise');

/** Define App Constants **/
if ( !defined('ROOT_PATH') )
    define('ROOT_PATH', dirname(dirname(__FILE__)));

if ( !defined('APP_PATH') )
    define('APP_PATH', ROOT_PATH . '/app/' );

if( !defined('URL') )
    define('URL', 'https://' . $_SERVER['SERVER_NAME']);

try {
    include ROOT_PATH . "/app/bootstrap.php";

    /** Dependencies **/
    $di = new Phalcon\DI\FactoryDefault();
    $app = new Bootstrap($di);
    echo $app->run(array());

} catch (\Phalcon\Exception $e) {
        echo $e->getMessage();
} catch (PDOException $e){
        echo $e->getMessage();
}

Any ideas on how can I debug this issue further?

Thanks