URLs for unilm vocab.txt are deprecated

[YebowenHu]

The vocab.txt links in tokenization_unilm.py are deprecated.
Need to update the latest checkpoints links according to [link].(https://unilm.blob.core.windows.net/ckpt/unilm1.2-base-uncased-vocab.txt)

PRETRAINED_VOCAB_FILES_MAP = {
    'vocab_file':
    {
        'unilm-large-cased': "https://conversationhub.blob.core.windows.net/beit-share-public/ckpt/unilm-large-cased-vocab.txt",
        'unilm-base-cased': "https://conversationhub.blob.core.windows.net/beit-share-public/ckpt/unilm-base-cased-vocab.txt",
        'unilm1-large-cased': "https://conversationhub.blob.core.windows.net/beit-share-public/ckpt/unilm1-large-cased-vocab.txt",
        'unilm1-base-cased': "https://conversationhub.blob.core.windows.net/beit-share-public/ckpt/unilm1-base-cased-vocab.txt",
        'unilm1.2-base-uncased': "https://conversationhub.blob.core.windows.net/beit-share-public/ckpt/unilm1.2-base-uncased-vocab.txt"
    }
}

Thus, links in configuration_minilm.py, configuration_unilm.py, tokenization_minilm.py, tokenization_unilm.py need to be updated as well.