text: Title:  I met this error: 
  Body: Hi, @jtoy @hunkim @Kongsea @DingKe @JayParks 
  
  I met this error:
  
  sgiO2:image_captioning sgi$ python build_vocab.py 
  loading annotations into memory...
  Traceback (most recent call last):
    File "build_vocab.py", line 77, in <module>
      main(args)
    File "build_vocab.py", line 59, in main
      threshold=args.threshold)
    File "build_vocab.py", line 31, in build_vocab
      coco = COCO(json)
    File "/Library/Python/2.7/site-packages/pycocotools/coco.py", line 84, in __init__
      dataset = json.load(open(annotation_file, 'r'))
  IOError: [Errno 2] No such file or directory: '/usr/share/mscoco/annotations/captions_train2014.json'
  
  What's wrong with me?
  