Update composer.json

Author: z-song

composer.json

@@ -13,12 +13,12 @@
     ],
     "require": {
         "php": ">=7.0.0",
-        "laravel/framework": "~5.6",
+        "laravel/framework": "~5.5",
         "encore/laravel-admin": "1.5.*"
     },
     "require-dev": {
         "phpunit/phpunit": "~6.0",
-        "laravel/laravel": "~5.6"
+        "laravel/laravel": "~5.5"
     },
     "autoload": {
         "psr-4": {