Update composer.json

Author: GrahamCampbell

composer.json

@@ -18,7 +18,7 @@
     },
     "require-dev": {
         "graham-campbell/analyzer": "^2.0",
-        "graham-campbell/testbench": "^4.0",
+        "graham-campbell/testbench": "^4.0|^5.0",
         "phpunit/phpunit": "^6.5"
     },
     "autoload": {