hydra: 2019-03-18 -> 2019-05-06

As Hydra doesn't have a release model, we have to pin git revs for our
package. The current state provides some new fixes and features like a status
plugin for GitLab.

As Nix 2.0 isn't bundled in `master` and `release-19.03` and we don't
support Nix 1.11 for Hydra, I figured that the Nix version checks could
be dropped and boost can always be used as build input.