GoString plugin: generate values instead of pointers when a field is repeated and non-nullable.

[godfried]

See #630

[godfried]

I haven't regenerated any of the other protos yet.

[jmarais]

Thanks for the pull request.

Currently there is only one test case for a repeated non-nullable message, can you please add another test for the repeated nullable case?

The travis check is currently failing because of the changes in the generated pb.go files. You will need to regenerate all the pb.go files.