vendor: bump github.com/coreos/go-systemd to v2

There are a couple of bugfixes since this was last bumped. Pull them in.

Docker-DCO-1.1-Signed-off-by: Brandon Philips <brandon.philips@coreos.com> (github: philips)
Merge pull request #5981 from philips/bump-go-systemd

vendor: bump github.com/coreos/go-systemd to v2