add laravel-backup as requirement

Author: songzou

composer.json

@@ -14,7 +14,8 @@
     "require": {
         "php": ">=7.0.0",
         "laravel/framework": "5.5.*",
-        "encore/laravel-admin": "1.5.*"
+        "encore/laravel-admin": "1.5.*",
+        "spatie/laravel-backup": "5.5.*"
     },
     "require-dev": {
         "phpunit/phpunit": "~6.0",