[Autoloading] Made the require_once of the UnviversalClassLoader conditional.

As we can't know what happened before we require the silex.phar file, we
can deduce if an autoloader has already been required.
It can lead to fatal errors for cases like SilexBundle.

autoload.php

 <?php
 
-require_once __DIR__.'/vendor/Symfony/Component/ClassLoader/UniversalClassLoader.php';
+if(false === class_exists('\Symfony\Component\ClassLoader\UniversalClassLoader')) {
+    require_once __DIR__.'/vendor/Symfony/Component/ClassLoader/UniversalClassLoader.php';
+}
 
 use Symfony\Component\ClassLoader\UniversalClassLoader;