sync multi-bleu.perl with moses version
sync multi-bleu.perl with moses version

(somehow, multi-reference support was removed at one point)

rsennrich/nematus,rsennrich/nematus,EdinburghNLP/nematus,EdinburghNLP/nematus,EdinburghNLP/nematus,rsennrich/nematus,rsennrich/nematus,rsennrich/nematus,EdinburghNLP/nematus,EdinburghNLP/nematus,rsennrich/nematus,EdinburghNLP/nematus