Update mikey179/vfsStream

Author: snapshotpl

composer.json

@@ -23,7 +23,7 @@
     },
     "require-dev": {
         "phpunit/phpunit": "^7.5.20",
-        "mikey179/vfsStream": "^1.6.4",
+        "mikey179/vfsStream": "^1.6.8",
         "slim/slim": "^3.0",
         "zendframework/zend-expressive": "^3.0",
         "zendframework/zend-expressive-fastroute": "^3.0.1",