bump atc
bump atc

[finishes #113028975]

Submodule src/github.com/concourse/atc dc84b5c..612524f:
  > cache EdgeTarget.rankOfFirstAppearance

Signed-off-by: Evan Short <5b1db704470d35a058f1f4a9e1a5a172a6ea69d3@pivotal.io>

zachgersh/concourse,concourse/concourse,cjcjameson/concourse,cjcjameson/concourse,concourse/concourse,concourse/concourse,zachgersh/concourse,cjcjameson/concourse,concourse/concourse,concourse/concourse,zachgersh/concourse