Allow testing with doctrine/common 2.6

composer.json

@@ -13,7 +13,7 @@
     ],
     "require": {
         "php": ">=5.3.2",
-        "doctrine/common": ">=2.4,<2.6-dev"
+        "doctrine/common": "~2.4"
     },
     "require-dev": {
         "phpunit/phpunit": "4.*",