Update bleu.py (#320)

Summary:
Modify Error message of bleu.
Fix the issue:  https://github.com/pytorch/fairseq/issues/284
Pull Request resolved: https://github.com/pytorch/fairseq/pull/320

Differential Revision: D12876721

Pulled By: myleott

fbshipit-source-id: df25885a94a584cbf4b86a1665e3e513c7eb8e9a

fairseq/bleu.py

@@ -13,7 +13,7 @@ try:
     from fairseq import libbleu
 except ImportError as e:
     import sys
-    sys.stderr.write('ERROR: missing libbleu.so. run `python setup.py install`\n')
+    sys.stderr.write('ERROR: missing libbleu.so. run `python setup.py build develop`\n')
     raise e