Pin Mongoose to version 3.8.3 to fix 'key $ne' error

Authentication could sometimes fail with the following error:
"Failed authentication: Error: Could not remove orphaned sessions: Error: key $ne must not start with '$'"

Seems related to Mongoose bug 1876:
https://github.com/LearnBoost/mongoose/issues/1876

As with the reporter of that bug, rolling back to Mongoose 3.8.3 eliminates
the error here as well.

Change-Id: I1a30bf556a7ff69e896352cf9d6e7df08f9675b1
Reported-by: Ken Kegg <wockychan@hotmail.com>