change phpunit version

Author: agrandiere

composer.json

@@ -1,8 +1,8 @@
 {
     "require-dev": {
-        "phpunit/phpunit": "~4.4.0@stable"
+        "phpunit/phpunit": "^5.7"
     },
     "require": {
         "guzzlehttp/guzzle": "~6.0"
     }
-}
+}