update stage1 to the latest systemd (v234)

rkt currently uses systemd-v231 (see coreos-common.mk) and #3787 is pushing an update to systend-v233. However, systemd-v234 is already released, so we should update to the last version at some point.

I didn't immediately update to the last version because of the following limitation as explained on https://github.com/rkt/rkt/pull/3787#issue-255578984:

This is due to a change in systemd-nspawn-v234: see: https://github.com/systemd/systemd/pull/6261/files#diff-a8646f412b928a4679f2ec702b55c12bR338. With this changes, it now calls 'StartTransientUnit' synchronously. Before, it was called asynchronously without checking for errors. This 'StartTransientUnit' method was added in systemd-v205 (via https://github.com/systemd/systemd/commit/c2756a684011) so it is not yet available in SemaphoreCI.

I contacted SemaphoreCI and they hopefully will have a new testing platform with Ubuntu 16 in the next few weeks. Once that platform is available, we could upgrade stage1 again without interruption in the tests. I don't think we care about support for systemd v204 if not for the tests.

@lucab suggested in https://github.com/rkt/rkt/pull/3787#issuecomment-327466414:

I don't want to put too much focus on legacy systems, but should we detect <205 and skip registration on that? I seem be understand the misbehavior is on systemd-nspawn registration. This should allow to sync to latest.