`skipInput` for RNN doesn't work for cuDNN 5.15 (but works for cuDNN 7.1.2)

[xukai92]

[xukai92]

FYI, I found this by counting the number of parameters

[denizyuret]

If we can find the documentation/spec for cuDNN 5.15 on how exactly skipInput behaves for that version we can write a warning or condition in rnn.jl. However this is not high priority unless a user is stuck without the ability to upgrade.