Pull back code to set up GOPATH for build

If there’s no GOPATH set up locally, Go won’t be able to find sub packages. This is due to changes in https://github.com/github/hub/commit/af9163b699b34ffa167ef99853ff2f7374d7b01a.
Merge pull request #716 from github/setup_go_path

Pull back code to set up GOPATH for build