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/golang.org/x/text/transform/transform.go

@@ -78,8 +78,8 @@ type SpanningTransformer interface {
 	// considering the error err.
 	//
 	// A nil error means that all input bytes are known to be identical to the
-	// output produced by the Transformer. A nil error can be be returned
-	// regardless of whether atEOF is true. If err is nil, then then n must
+	// output produced by the Transformer. A nil error can be returned
+	// regardless of whether atEOF is true. If err is nil, then n must
 	// equal len(src); the converse is not necessarily true.
 	//
 	// ErrEndOfSpan means that the Transformer output may differ from the