bump atc
bump atc

concourse/concourse#2519

Submodule src/github.com/concourse/atc a96b3038..40d1071a:
  > add signing key to atc under test

Signed-off-by: Jamie Klassen <d740210502544aa0776b92978c6db85774130b86@pivotal.io>

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