bump atc
bump atc

Submodule src/github.com/concourse/atc 3687a53..62bda9e:
  > really centered favicons for real this time

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