Adding unit test for "current user in pre/post save hooks" feature

This is a simple unit test for the "current user in pre/post save hooks" feature (#514).

I tested the two scenarios we discussed
* when `UpdateHandler()` is used
* when `item.save()` is used

In the `UpdateHandler()` scenario, we expect the `_req_user` to be passed to the pre/post save hooks, while in the `item.save()` scenario we do not.

I took the liberty to add the `mongodb` service to the Travis-CI configuration file, which I required for this test.

Will continue my work on the PR for the `track` option tomorrow.