Bump monolog requirement

Author: Seldaek

composer.json

@@ -21,7 +21,7 @@
         "symfony/dependency-injection": "~2.3",
         "symfony/config": "~2.3",
         "symfony/http-kernel": "~2.3",
-        "monolog/monolog": "~1.3"
+        "monolog/monolog": "~1.6"
     },
     "require-dev": {
         "symfony/yaml": "~2.3",