bump atc
bump atc

Submodule src/github.com/concourse/atc 842c4aba..dd3361e4:
  > Only initialize CredHub client once

Signed-off-by: Nader Ziada <61306b0d4f20cfca7d1222440ff271187de40866@pivotal.io>

concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse