Add phpunit 6.0 fallback version

Author: slavcodev

composer.json

@@ -15,7 +15,7 @@
   "require": {
     "php": "^7.1",
     "psr/http-message": "^1.0",
-    "phpunit/phpunit": "^7.0",
+    "phpunit/phpunit": "^6.0 | ^7.0",
     "justinrainbow/json-schema": "~2.0"
   },
   "autoload": {