Artifact: Restart Timing (#68)

* Fire the migrate and restart callbacks after the current symlink is
  generated.
* This prevents a "Catch-22", chicken vs. egg problem, wherein the
  symlink referring to an executable in the "current" release points
  to the previous release instead of the release that has just been
  deployed.
* Fixes [upstream issue GitHub #68](https://github.com/RiotGames/artifact-cookbook/issues/68)