bump atc
bump atc

Submodule src/github.com/concourse/atc 5687d1afc..7ca815838:
  > take web from master

Signed-off-by: Maria Shaldibina <dc03896a0185453e5852a5e0f0fab6cbd2e026ff@pivotal.io>

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