fix composer json

Author: tck

composer.json

@@ -35,7 +35,7 @@
     },
     "require-dev": {
         "phpunit/phpunit": "~4.3|~5",
-        "mikey179/vfsStream": "1.*",
+        "mikey179/vfsstream": "1.*",
         "squizlabs/php_codesniffer": "1.*",
         "phploc/phploc": "2.*",
         "pdepend/pdepend": "2.*",