This project currently depends on a version of github.com/coreos/go-systemd that didn't use Go modules. This led to the following error when doing a go get -u all ./...:

go: finding github.com/coreos/go-systemd latest
go get all: no matching versions for query "upgrade" (current version is v0.0.0-20190321100706-95778dfbb74e)

I think this could be resolved by updating to v22, which uses Go modules.