Don't track the /third_party directory

This is used by the CoreOS dependency management tool we're going to
use.

Link: https://github.com/coreos/third_party.go
Use the third_party.go tool from CoreOS for deps

This is being used as we don't want to explicitly set a GOPATH for our
CI environment and may want to (at a later date) vendor our third
party dependencies directly into the git repo.