Add missing require for symfony/yaml in composer.json

Author: Sebastian Dudek

composer.json

@@ -17,7 +17,8 @@
         "php": ">=5.3.0",
         "symfony/intl": "~2.3",
         "symfony/config": "~2.3",
-        "symfony/translation": "~2.3"
+        "symfony/translation": "~2.3",
+        "symfony/yaml": "~2.3"
     },
     "require-dev": {
         "phpspec/phpspec": "2.0.*"