bump atc
bump atc

Submodule src/github.com/concourse/atc 39c4d06..5f42d96:
  > don't panic when a StaticConfigSource has no config
  > remove lying comment

Signed-off-by: Kris Hicks <6d3f751023bc04266d4da2ff94804f4240839fe2@pivotal.io>

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