Updates mongo to the same version used by performance platform
  - This fixes a bug for us which is causing tests to fail:
    https://ci-new.alphagov.co.uk/job/performanceplatform_backdrop/1115/console
  - Details of the bug in versions 2.4.3-6 are here https://jira.mongodb.org/browse/SERVER-9501
Updating slave 4 at request of @mattbostock for consistency
Change mongodb-2.4.9 label to mongodb-2.4

Do not encourage apps to rely on too specific a mongodb version.
Merge pull request #120 from alphagov/update_mongo_for_the_poorformance_platform

Updates mongo to the same version used by performance platform