[WebBundle] fixed Symfony version in the text exception message

src/Symfony/Framework/WebBundle/Resources/views/Exception/exception.txt.php

@@ -1,5 +1,7 @@
 [exception]   <?php echo $code.' | '.$text.' | '.$name ?>
+
 [message]     <?php echo $message ?>
+
 <?php if (isset($traces) && count($traces) > 0): ?>
 [stack trace]
 <?php foreach ($traces as $line): ?>
@@ -7,5 +9,5 @@
 
 <?php endforeach; ?>
 <?php endif; ?>
-[symfony]     v. <?php echo SYMFONY_VERSION ?> (symfony-project.org)
+[symfony]     v. <?php echo \Symfony\Foundation\Kernel::VERSION ?> (symfony-project.org)
 [PHP]         v. <?php echo PHP_VERSION ?>