Use pug 3 stable

Author: kylekatarnls

composer.json

@@ -3,7 +3,7 @@
   "description": "Pug template assets manager",
   "type": "library",
   "require": {
-    "pug-php/pug": "^2.0.0 || ^3.0.0@beta",
+    "pug-php/pug": "^2.0.0 || ^3.0.0",
     "pug-php/pug-minify": "^1.0.1",
     "pug-php/pug-filter-coffee-script": "^1.2",
     "pug-php/pug-filter-less": "^1.0",