Fix sacrebleu (#630)

Summary:
Pull Request resolved: https://github.com/pytorch/fairseq/pull/630

sacrebleu scorer has stopped working in pytorch_translate (maybe
fairseq too) probably due to  a recent api change.

Reviewed By: jmp84

Differential Revision: D14792797

fbshipit-source-id: c2a00246e08bc913c41e60c5fbf8ab4ab5e80d18