Downgrade to phpunit 7 to support php 7.1

Author: pchasle

composer.json

@@ -31,15 +31,18 @@
   },
   "require-dev": {
     "friendsofphp/php-cs-fixer": "^2.14",
-    "phpunit/phpunit": "^8.0",
+    "phpunit/phpunit": "^7.0",
     "phpspec/phpspec": "^5.0",
     "symfony/yaml": "^4.2",
     "donatj/mock-webserver": "^2.0",
     "http-interop/http-factory-guzzle": "^1.0",
     "php-http/guzzle6-adapter": "^2.0"
   },
   "config": {
-    "bin-dir": "bin"
+    "bin-dir": "bin",
+    "platform": {
+      "php": "7.1.3"
+    }
   },
   "suggest": {
     "php-http/guzzle6-adapter": "In order to use Guzzle v6 as the HTTP client"