Added `symfony/expression-language` to composer.json

Tests are failing:
````
Fatal error: Class 'Symfony\Component\ExpressionLanguage\ExpressionLanguage' not found in /Volumes/www/test/JMSDiExtraBundle/vendor/symfony/security/Symfony/Component/Security/Core/Authorization/ExpressionLanguage.php on line 22
````

Added `symfony/expression-language` to composer.json to fix it.