bump topgun
bump topgun

[finishes #129725995]

Submodule src/github.com/concourse/topgun 452713e..ec60e07:
  > add tests for build containers gc

Signed-off-by: Alex Suraci <a8053324ea51f0cc533cfe7b17f8bee3c920919f@pivotal.io>

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