Update mongodb to the latest version 🚀 (#4855)

* fix(package): update mongodb to version 3.1.0

* chore(package): update lockfile

https://npm.im/greenkeeper-lockfile

* starting mongo 3.1.0, read preferences are passed again

* Adds test confirming #4831 is properly functional now

package.json

@@ -32,7 +32,7 @@
     "lodash": "4.17.5",
     "lru-cache": "4.1.2",
     "mime": "2.3.1",
-    "mongodb": "3.0.7",
+    "mongodb": "3.1.0",
     "multer": "1.3.0",
     "parse": "1.11.1",
     "pg-promise": "8.4.0",