Work around packagist bug

Author: GrahamCampbell

composer.json

@@ -18,7 +18,7 @@
         "orchestra/support-providers": "3.0.*"
     },
     "require-dev": {
-        "graham-campbell/testbench": "~2.0"
+        "graham-campbell/testbench": "dev-master"
     },
     "autoload": {
         "psr-4": {