issue #13 - increase minimal PHP version

JsonSerializable interface is only available since PHP >= 5.4.0

Author: mikaelcom

composer.json

@@ -21,7 +21,7 @@
         "email" : "[email protected]"
     },
     "require": {
-        "php" : ">=5.3.3"
+        "php" : ">=5.4"
     },
     "require-dev": {
         "friendsofphp/php-cs-fixer": "~2.0",