chore(package): update mongodb to version 2.2.21

https://greenkeeper.io/

Author: greenkeeperio-bot

package.json

@@ -27,7 +27,7 @@
     "lodash": "4.17.4",
     "lru-cache": "4.0.2",
     "mime": "1.3.4",
-    "mongodb": "2.2.10",
+    "mongodb": "2.2.21",
     "multer": "1.2.1",
     "parse": "1.9.2",
     "parse-server-fs-adapter": "1.0.1",