Update Node.js

0.10.21 has been added to the CI PPA.

I'm hoping `apt-get update` [will have run already](https://github.com/alphagov/ci-puppet/blob/1604a583b9fa35d6914abb4a126c4d791a45eb75/modules/ci_environment/manifests/base.pp#L70).

See also: https://github.com/alphagov/pp-puppet/pull/81