allow OptionsResolver 2.3

Author: Christoph Singer

composer.json

@@ -6,7 +6,7 @@
     "require": {
         "mrclay/jsmin-php": "^2.3",
         "tubalmartin/cssmin": "^2.4",
-        "symfony/options-resolver": "^2.6"
+        "symfony/options-resolver": "^2.3"
     },
     "autoload":{
         "psr-4":{