Upgrade mongo as per the other slaves

This was pinned to mongo 2.0.9 because this was required for the Licensify tests. However, Licensify is running on 2.4.9 in production and the tests pass on 2.4.9 so this is not required.
Move mongo version to generic slave definition

Since this is now the same version for all slaves, it only needs to be defined in one place.
Merge pull request #217 from alphagov/licensify-updated-mongo

Upgrade mongo as per the other slaves