bump locket
bump locket

Submodule src/code.cloudfoundry.org/locket 78c84d9a..f5349082:
  > fix lockets unit test when reading metrics from a channel

Signed-off-by: Chris Piraino <f047703f03f13cc8e1b226aa8ba85c2dfc238f94@pivotal.io>

cloudfoundry-incubator/diego-release,cloudfoundry-incubator/diego-release,cloudfoundry-incubator/diego-release,cloudfoundry-incubator/diego-release