bump atc
bump atc

[finishes #115901437]

Submodule src/github.com/concourse/atc 2f9a4ad..f3735e6:
  > new favicon

Signed-off-by: Kim Eberz <54f6dba7fc2457760a735c390848ac756e4e6baa@gmail.com>

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