EDIT:
sorry this is just a user warning
EDIT #2:
you dont need to wrap your tensors in Variables anymore. Since 0.4 Variables and Tensors are merged. I recommend you take a loot at the migration guide 51 to make your code cleaner and compatible with future releases 
I think the problem is, you are using dropout with only one layer. You need at least 2 layers to apply dropout if you are using the LSTM class. You can check the documentation here 15. It says:

dropout – If non-zero, introduces a Dropout layer on the outputs of each LSTM layer except the last layer, with dropout probability equal to dropout. Default: 0

best regards,
Diego