Only run CI build on instances with Mongodb 2.4
Only run CI build on instances with Mongodb 2.4

This commit ensures that the build runs on nodes with Mongodb 2.4
installed which should fix #898.

Some of the CI 'agent' nodes on the Jenkins server run a more recent
version of mongodb. By experimenting with rebuilding on Jenkins we've
noticed that when this application's build runs on `ci-agent-4`, which
is configured[1] to run mongodb 3.2 it fails[2], when it runs on
`ci-agent-1` through `ci-agent-3` it seems to pass.

Fixes: #898

[1] https://github.com/alphagov/govuk-puppet/blob/master/hieradata/common.yaml#L639-L654
[2] https://github.com/alphagov/manuals-publisher/issues/898#issuecomment-288057880

alphagov/manuals-publisher,alphagov/manuals-publisher,alphagov/manuals-publisher