bump nsync
bump nsync

[finishes #109489966]

Submodule src/github.com/cloudfoundry-incubator/nsync 8a51091..b878e88:
  > only build execution metadata when ports are not specified on the desired app
  > Merge remote-tracking branch 'origin/pr/4'

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