all: fix travis after v2 update

Change-Id: I1c8d926a857beef239973e762f00d324a4da3b4d
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/166890
Reviewed-by: Joe Tsai <[email protected]>

Author: dsnet

jsonpb/jsonpb_test_proto/more_test_objects.pb.go

@@ -64,9 +64,7 @@ func main() {
 			if !f.Generate {
 				continue
 			}
-			filename := f.GeneratedFilenamePrefix + ".pb.go"
-			g := gen.NewGeneratedFile(filename, f.GoImportPath)
-			gengo.GenerateFile(gen, f, g)
+			g := gengo.GenerateFile(gen, f)
 			if grpc {
 				gengogrpc.GenerateFileContent(gen, f, g)
 			}