Merge pull request #81 from Kryniol/fix-80-phpunit-version-bump

PHPUnit version 6

Author: Kryniol

composer.json

@@ -34,7 +34,7 @@
         "guzzlehttp/psr7": "^1.3"
     },
     "require-dev": {
-        "phpunit/phpunit": "5.5.*",
+        "phpunit/phpunit": "^6.5",
         "friendsofphp/php-cs-fixer": "^2.0"
     },
     "suggest": {