Bump the Mongoid dependency to 2.5.x onwards.

Our current Mongo drivers are 15 months out of date, which may be
showing up a bug with the connection pool handling. Version 2.5.0 and
onwards of Mongoid support the newer drivers. Versions 2.6.x and 2.7.x
claim to be backwards-compatible, so we don't want to stop apps using
those.
Merge pull request #98 from alphagov/bump-mongoid-dependency

Bump the Mongoid dependency to 2.5.x onwards.