Yesterday we had a discussion about several Hydra-related problems in the #nixos
IRC channel[1], as a result a lot of stalled PRs were merged.
I think that we want to use the latest Hydra now as it contains lots of
new fixes and folks mentioned that they want to use a self-hosted
Hydra, so they probably use the `nixpkgs` package rather than the
expressions from `NixOS/hydra`.
Simply moving to 2019-03-18 should be fine for now as Hydra doesn't have
a release model.
[1] https://logs.nix.samueldr.com/nixos/2019-03-17#2048416;
341aeb8cf3