afmongodb: Update the internal libmongo-client version

We need a new version of the internal libmongo-client, to fix an issue
with safe mode. This fixes #95.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>
Merge pull request #108 from algernon/h/mongodb/libmongo-client-update

afmongodb: Update the internal libmongo-client version