[HttpKernel] Fix event dispatcher dependency

Update composer.json to account for #9792

Author: hacfi

composer.json

@@ -17,7 +17,7 @@
     ],
     "require": {
         "php": ">=5.3.3",
-        "symfony/event-dispatcher": "~2.1",
+        "symfony/event-dispatcher": "~2.5",
         "symfony/http-foundation": "~2.4",
         "symfony/debug": "~2.5",
         "psr/log": "~1.0"