bump atc
bump atc

[finishes #119388165]

Submodule src/github.com/concourse/atc 47c5128..affbc9e:
  > use base handler for !(GET/HEAD) HTTP traffic w/ TLS

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