bump atc
bump atc

[finishes #137641079]

Submodule src/github.com/concourse/atc ad0576b..5967137:
  > Stop tracking builds when you are being shut down [#137641079]

Signed-off-by: Chris Hendrix <f9e38fc3e1ff46784bdcadbf77d17f93932faf2c@pivotal.io>

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