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/google.golang.org/grpc/encoding/encoding.go

@@ -102,10 +102,10 @@ func RegisterCodec(codec Codec) {
 	if codec == nil {
 		panic("cannot register a nil Codec")
 	}
-	contentSubtype := strings.ToLower(codec.Name())
-	if contentSubtype == "" {
-		panic("cannot register Codec with empty string result for String()")
+	if codec.Name() == "" {
+		panic("cannot register Codec with empty string result for Name()")
 	}
+	contentSubtype := strings.ToLower(codec.Name())
 	registeredCodecs[contentSubtype] = codec
 }