Use more laxed phpunit versioning

Author: Riimu

composer.json

@@ -20,7 +20,7 @@
     "psr/http-message": "^1.0"
   },
   "require-dev": {
-    "phpunit/phpunit": "^6.0",
+    "phpunit/phpunit": "^5.7 || ^6.0",
     "squizlabs/php_codesniffer": "^2.8",
     "friendsofphp/php-cs-fixer": "^2.1"
   },