Fixed issue when running on PHP 7.1

Author: Joao Jacome

composer.json

@@ -24,7 +24,7 @@
     },
     "require-dev": {
         "phpunit/phpunit": "^7.0.1",
-        "mikey179/vfsStream": "^1.6",
+        "mikey179/vfsstream": "^1.6",
         "php-coveralls/php-coveralls" : "v2.0.0"
     },
     "autoload": {