Deploy from a 'machine user'

[nacnudus]

Currently Travis deloys to the gh-pages branch using a personal account. We should create a new account to be used for this kind of thing.

https://developer.github.com/v3/guides/managing-deploy-keys/#machine-users

Also needs documentation here and maybe in https://github.com/ukgovdatascience/govdown.

Thanks @tjtnew for reporting.