Update browserkit dependency to 2.0.x-dev

2.0.x-dev of browser-kit-testing package works for
Laravel 5.5.* and php 7.0

composer.json

@@ -22,7 +22,7 @@
     "require": {
         "php": ">=7.0.0",
         "illuminate/support": "5.5.* || 5.6.* || 5.7.*",
-        "laravel/browser-kit-testing": "^2.0 || ^4.0"
+        "laravel/browser-kit-testing": "2.0.x-dev || ^4.0"
     },
     "require-dev": {
         "orchestra/testbench": "~3.0"