Fix Laravel 6 dependency (#55)

Author: gjm

composer.json

@@ -11,7 +11,7 @@
     ],
     "require": {
         "php": "^7.2",
-        "laravel/framework": "6.0.*"
+        "laravel/framework": "^6.0"
     },
     "autoload": {
         "psr-4": {