bump atc
bump atc

[finishes #108151440]

Submodule src/github.com/concourse/atc 78268d6..630d76c:
  > emit initialize-get + initialize-put events

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

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