Allow monolog/monolog ^2.0 to be required as dependency

Author: mrtus

composer.json

@@ -15,7 +15,7 @@
     ],
     "require": {
         "php": ">=5.5",
-        "monolog/monolog": "^1.6",
+        "monolog/monolog": "^1.6|^2.0",
         "psr/log": "^1.0"
     },
     "require-dev": {