Update composer.json to allow symfony 3

Author: fterrier

composer.json

@@ -13,8 +13,8 @@
     "require": {
         "ext-curl": "*",
         "beberlei/assert": "2.*",
-        "symfony/yaml": "2.*",
-        "symfony/event-dispatcher": "~2.1"
+        "symfony/yaml": "2.*|3.*",
+        "symfony/event-dispatcher": "~2.1|3.*"
     },
 
     "require-dev": {