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.
Merge pull request #147 from alphagov/upgrade_slave-1_mongo

Upgrade mongodb on slave-1