(RE-3596, RE-3608) Allow local directory to be overridden for ship

This commit updates the ship task to allow a directory other than "pkg"
be used for storing artifacts for ship. It sets the default to be pkg,
so that current users should be unaffected.
Merge pull request #363 from haus/ticket/master/re-3597-use-rake-from-ruby

(RE-3596, RE-3608) Allow local directory to be overridden for ship