Fix travis

Author: norkunas

composer.json

@@ -18,7 +18,7 @@
         "symfony/options-resolver": "^3.4|^4.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "^7.5",
+        "phpunit/phpunit": "^5.7",
         "php-http/guzzle6-adapter": "^1.1"
     },
     "autoload": {