NUR: Updates can now be pushed

[Jörg Thalheim]

Before updates are propagated in NUR[1], we check for evaluation errors.
Until now we checked for updates on a daily base using a travis cron job.
Now there is an update service to trigger updates more frequently: https://
github.com/nix-community/NUR#update-nurs-lock-file-after-updating-your-
repository

Also checkout the source code here: https://github.com/nix-community/nur-update

[1] https://github.com/nix-community/NUR