[2.3] Fix lowest deps

Author: nicolas-grekas

composer.json

@@ -19,7 +19,7 @@
         "php": ">=5.3.3",
         "symfony/event-dispatcher": "~2.1",
         "symfony/http-foundation": "~2.3,>=2.3.4",
-        "symfony/debug": "~2.3,>=2.3.24",
+        "symfony/debug": "~2.3.24|~2.5.9|~2.6,>=2.6.2",
         "psr/log": "~1.0"
     },
     "require-dev": {