Support PHPUnit 7

Author: sagikazarmark

composer.json

@@ -16,7 +16,7 @@
     ],
     "require": {
         "php": ">=5.4",
-        "phpunit/phpunit": "^4.8.35 || ^5.4.3 || ^6.0",
+        "phpunit/phpunit": "^4.8.35 || ^5.4.3 || ^6.0 || ^7.0",
         "php-http/httplug": "^1.0 || ^2.0",
         "php-http/message": "^1.0",
         "guzzlehttp/psr7": "^1.0",