Pin mongodb package

Fixes #1855

Author: drew-gross

package.json

@@ -31,7 +31,7 @@
     "lru-cache": "^4.0.0",
     "mailgun-js": "^0.7.7",
     "mime": "^1.3.4",
-    "mongodb": "~2.1.0",
+    "mongodb": "2.1.18",
     "multer": "^1.1.0",
     "parse": "^1.8.0",
     "parse-server-fs-adapter": "^1.0.0",