bump atc
bump atc

[finishes #97774988]

Submodule src/github.com/concourse/atc f8f4221..db2aa26:
  > handle empty versions from resource steps
  > tweak display of steps that will never be run
  > darken first-occurrence bg and restore fg color

Signed-off-by: Chris Brown <eb0819782e2e967e3d4f4d71eab0c647ed2659cf@pivotal.io>
Signed-off-by: Robert Neumann <0e50484e9096968d17f79de0206b195d065c4510@pivotal.io>

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