switch to PHP5.6 as PHP5.5 is out of support since 10.07.2016

Author: eugene-matvejev

composer.json

@@ -21,7 +21,7 @@
         }
     },
     "require": {
-        "php": ">= 5.5.9",
+        "php": ">= 5.6",
         "leafo/lessphp": "^0.5",
         "leafo/scssphp": "@dev",
         "leafo/scssphp-compass": "@dev"