Make sure we dont include multiple vendor folders (#934)

* Make sure we dont include multiple vendor folders * Require discovery 1.6

Author: Nyholm

composer.json

@@ -19,7 +19,7 @@
  "php-http/promise": "^1.0"
  },
  "require-dev": {
- "phpunit/phpunit": "6.3.*",
+ "phpunit/phpunit": "^6.5 || ^7.5",
  "cache/void-adapter": "^1.0"
  },
  "autoload": {