tag source code with current semver when promoting build
tag source code with current semver when promoting build

[#142553399](https://www.pivotaltracker.com/story/show/142553399)

Signed-off-by: Danny Berger <8efccf38e9d8f361e1973ad644e1b1e50ad8ecfe@pivotal.io>

cloudfoundry/bosh-agent,mattcui/bosh-agent,gu-bin/bosh-agent,gu-bin/bosh-agent,gu-bin/bosh-agent,mattcui/bosh-agent,cloudfoundry/bosh-agent,mattcui/bosh-agent