Add phpunit 7

Author: wissem

composer.json

@@ -19,7 +19,7 @@
         "php-http/client-common": "^1.9 || ^2.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "^5.5 || ^6.0",
+        "phpunit/phpunit": "^5.5 || ^7.0",
         "php-http/guzzle6-adapter": "^1.0 || ^2.0",
         "php-http/mock-client": "^1.0",
         "guzzlehttp/psr7": "^1.2",