Bump version to v1.3.8 for CoreOS Stable 494.5.0

• Based on CoreOS Stable 494.5.0
  • kernel v3.17.2
  • etcd v0.4.6
  • fleet v0.8.3
  • docker v1.3.3
  • coreos-cloudinit v0.10.9
  • systemd 215