bump atc
bump atc

Submodule src/github.com/concourse/atc ef9b6574..e397ba63:
  > scale down dashboard

Signed-off-by: Gabriel Dumitrescu <ac98dc248ba95b80e235d49d71a9a50afc4ea93d@pivotal.io>

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