Add support for Laravel 6 (#12)

* add support for laravel 6
* fix dependencies for testing using laravel 6

Author: nevoxx

composer.json

@@ -22,11 +22,11 @@
     "require": {
         "php": "^7.1",
         "0.0.0/composer-include-files": "^1.5",
-        "illuminate/support": "^5.6"
+        "illuminate/support": "^5.6|^6.0"
     },
     "require-dev": {
-        "orchestra/testbench": "3.6.*",
-        "phpunit/phpunit": "^7.0"
+        "orchestra/testbench": "3.6.*|^4.0",
+        "phpunit/phpunit": "^7.0|^8.0"
     },
     "scripts": {
         "test": "phpunit"