bump atc
bump atc

[finishes #128520085]

Submodule src/github.com/concourse/atc 62bfa76..494644b:
  > add logout endpoint
  > fix spacing on login button

Signed-off-by: Chris Dutra <7eb54fc324f04033595eb420f2ee7c112fb7259e@pivotal.io>

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