fixed composer format

mixed caps packages are deprecated

Author: splitbrain

composer.json

@@ -21,7 +21,7 @@
 
     "require-dev": {
         "phpunit/phpunit": "^4.8",
-        "mikey179/vfsStream": "^1.6",
+        "mikey179/vfsstream": "^1.6",
         "ext-zip": "*",
         "ext-bz2": "*"
     },