Fix setting of updated_at field.

This wasn't being set because the `Mongoid::Timestamps` module doesn't
create the necessary `before_upsert` hooks.  Manually adding this
callback fixes this issue.  I've also added an integration level test to
ensure this doesn't fail again in future.

See https://github.com/mongoid/mongoid/issues/3716 for the bug report
against mongoid
Merge pull request #26 from alphagov/fix_updated_at

Fix setting of updated_at field.