bump atc
bump atc

concourse/concourse#2452

Submodule src/github.com/concourse/atc 0b534f318..7f08cd0cd:
  > use own migration library for upgrade
  > Remove use of mattes/migrate library from db migrations
  > remove yeller configuration flags
  > Merge pull request #294 from concourse/upsert
  > Add a failing test for duplicated WRCCS
  > Add pinned version to resource on pipeline config

Signed-off-by: Divya Dadlani <509f062e20590ad33545b76914b972e08ef4d2e0@pivotal.io>

concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse