It turned out that changes like
kubernetes-csi/csi-lib-utils#33 should better
have been committed after `go mod tidy` because that adds some
indirect dependencies in that example.

The revised `test-vendor` checks for that and (just in case that this
ever becomes desired) allows projects to not have a vendor directory
when using `go mod`.

How to use `go mod` properly gets documented in the README.md, because
there are such pitfalls.

This script handles the necessary "replace" statements and determines
which packages need to be updated in lockstep.

Signed-off-by: Deep Debroy <ddebroy@docker.com>

Build Windows only for amd64

update Go mod support

errors when the depth is exactly 50.

Do full git clones in travis

A simple "go mod init && go mod tidy" did not pick compatible versions
of the k/k modules, so go-get-kubernetes.sh had to be used, which in
turn implied that a new Kubernetes release with go.mod file had to be
picked, because that is needed by the script.