fix(sys-apps/systemd): Stabilize linux headers 3.13 for systemd

The currently systemd git only compiles against 3.10 headers or later.
Looks like upstream Gentoo is going to stabilize 3.13 so go ahead and
do jump ahead on that to make testing systemd easier.

Depends on https://github.com/coreos/portage-stable/pull/107

This should not be merged until after the first beta release just to avoid unnecessary risk.