Fix protoc-gen-go tests (#371)

Golden was not updated after
https://github.com/golang/protobuf/commit/47eb67eaf5cab63c58956d4d4ce86b03ad5eaa03

ref #329

protoc-gen-go/testdata/my_test/test.pb.go.golden

@@ -1,6 +1,5 @@
-// Code generated by protoc-gen-go.
+// Code generated by protoc-gen-go. DO NOT EDIT.
 // source: my_test/test.proto
-// DO NOT EDIT!
 
 /*
 Package my_test is a generated protocol buffer package.