more compatible

the change can avoid the failure of loading library file. Image there is index.php file which load the library siting in different folder from the library vero-php, like the follow structure:
- index.php
- library
     - vero-php
        - vero
           - client.php
        - vero.php

Index.php can't load the file using relative path as require('./vero/client.php'). A more way proper way would be require(dirname(__FILE__) . '/vero/client.php').