[Serializer] polyfill for json_last_error_msg is in php 5.5

Author: Tobion

composer.json

@@ -17,7 +17,7 @@
     ],
     "require": {
         "php": ">=5.3.9",
-        "symfony/polyfill-php54": "~1.0"
+        "symfony/polyfill-php55": "~1.0"
     },
     "require-dev": {
         "symfony/yaml": "~2.0,>=2.0.5|~3.0.0",