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

@@ -19,10 +19,10 @@
         "php-http/message-factory": "^1.0.2",
         "psr/http-message-implementation": "^1.0",
         "php-http/client-implementation": "^1.0",
-        "php-http/discovery": "^1.4"
+        "php-http/discovery": "^1.6"
     },
     "require-dev": {
-        "phpunit/phpunit": "6.3.*",
+        "phpunit/phpunit": "^6.5 || ^7.5",
         "symfony/stopwatch": "~2.5",
         "php-http/message": "^1.0",
         "php-http/mock-client": "^1.0",