bump topgun
bump topgun

[finishes #134489111]

Submodule src/github.com/concourse/topgun d004f95..b920a3b:
  > fix the hijacking test to be interruptible

Signed-off-by: Chris Hendrix <f9e38fc3e1ff46784bdcadbf77d17f93932faf2c@pivotal.io>

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