[composer] Loosen symfony/event-dispatcher constraint

composer.json

@@ -13,7 +13,7 @@
     ],
     "require": {
         "php": ">=5.3.0",
-        "symfony/event-dispatcher": "v2.0.15"
+        "symfony/event-dispatcher": "2.*"
     },
     "autoload": {
         "psr-0": { "Solarium": "library/" }