Upgrade mongodb on slave-1

Now that most things (everything?) has been upgraded to mongo2.4 in
prod, it makes sense to have most of out CI boxes also running 2.4.

This leaves slave-3 running 2.0 for now so that any jobs still tied to
2.0 will continue to run.  That can beupgraded when there are no more
jobs that depend on 2.0.