Remove camelCase package name from composer.json

Author: Steven Weingärtner

composer.json

@@ -14,7 +14,7 @@
         "php": "^7.1"
     },
     "require-dev": {
-        "mikey179/vfsStream": "^1.2",
+        "mikey179/vfsstream": "^1.2",
         "phpunit/phpunit": "^6.0",
         "symfony/http-foundation": "^2.8|^3.0",
         "symfony/security-core": "^2.8|^3.0"