update vfsStream to 1.6.*

Author: ek9

composer.json

@@ -37,7 +37,7 @@
     },
     "require-dev": {
         "phpunit/phpunit": "~5.0",
-        "mikey179/vfsStream": "1.3.*"
+        "mikey179/vfsStream": "1.6.*"
     },
     "suggest": {
         "ext-xdebug": "Xdebug extension is required to collect coverage via Xdebug driver and run tests"