fix uppercase chars in package names (compatibility to Composer 2)

composer.json

@@ -38,6 +38,6 @@
     },
     "require-dev": {
         "phpunit/phpunit": "~5.7",
-        "mikey179/vfsStream": "^1.6"
+        "mikey179/vfsstream": "^1.6"
     }
 }