bump atc
bump atc

[finishes #128029305]

Submodule src/github.com/concourse/atc 6b60616..dc4ffc1:
  > if connection is dead reconnect
  > lock db connection on Acquire and Release

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

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