OpenShift and Kubernetes use Godep for dependency management. Godep allows versions of dependent packages to be locked at a specific commit by vendoring them (checking a copy of them into Godeps/_workspace/). This means that everything you need for OpenShift is checked into this repository, and the hack/config-go.sh script will set your GOPATH appropriately. To install godep locally run:
$ go get github.com/tools/godep
If you are not updating packages you should not need godep installed.
To update to a new version of a dependency that's not already included in Kubernetes, checkout the correct version in your GOPATH and then run godep save <pkgname>. This should create a new version of Godeps/Godeps.json, and update Godeps/workspace/src. Create a commit that includes both of these changes.
To update the Kubernetes version, checkout the new "master" branch from openshift/kubernetes (within your regular GOPATH directory for Kubernetes), and run godep restore ./... from the Kubernetes dir. Then switch to the OpenShift directory and run godep save ./...