Do not allow installation in Backpack 4.0

Author: tabacitu

composer.json

@@ -12,7 +12,7 @@
     "homepage": "https://github.com/laravel-backpack/filemanager",
     "keywords": ["Laravel", "FileManager"],
     "require": {
-        "backpack/crud": "^4.0.0|^4.1.0",
+        "backpack/crud": "^4.1.0|4.1.x-dev",
         "barryvdh/laravel-elfinder": "^0.4.5"
     },
     "require-dev": {