element_mask is moved

[kelizhong]

element_mask is moved in mxnet new version https://github.com/dmlc/mxnet/blob/master/docs/architecture/release_note_0_9.md

And I failed to run your code, do you have any suggestion to replace element_mask

The code is in https://github.com/freddycct/seq2seq/blob/master/lstm.py#L74