fixed typo

src/Silex/Extension/TwigExtension.php

@@ -32,7 +32,7 @@ class TwigExtension implements ExtensionInterface
                 array(
                     'charset'          => $app['charset'],
                     'debug'            => $app['debug'],
-                    'strict_variables' => !$app['debug'],
+                    'strict_variables' => $app['debug'],
                 ),
                 isset($app['twig.options']) ? $app['twig.options'] : array()
             );