Update to use go modules (#671)

This includes updates to CI config, vendor files, etc.
I've hard-coded the k8s depedency at 1.13.4 to keep it inline with what
we currently have and to make sure a another run of `go mod tidy`
doesn't accidentally update it to an unexpected version.

Thanks to hectorj2f for carrying this along.

vendor/github.com/golang/protobuf/proto/decode.go

@@ -186,7 +186,6 @@ func (p *Buffer) DecodeVarint() (x uint64, err error) {
 	if b&0x80 == 0 {
 		goto done
 	}
-	// x -= 0x80 << 63 // Always zero.
 
 	return 0, errOverflow