Improve phpstan package version handling

Author: shochdoerfer

composer.json

@@ -13,7 +13,7 @@
   ],
   "require": {
     "php": "^7.1.0",
-    "phpstan/phpstan": ">=0.11.6"
+    "phpstan/phpstan": "^0.11.6"
   },
   "require-dev": {
     "captainhook/captainhook": "^4.0",