bump atc
bump atc

[finishes #128825313]

Submodule src/github.com/concourse/atc 662fdb9..9a0d50d:
  > fix build output persisting when switching around
  > fix history build entry starting before header

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