Incorrect bprop() for LRN layer

[donglaiw]

In deeppy/feedforward/convnet_layers.py, the bprop for the LRN layer should not be the direct copy from the previous layer.

Reference: decaf https://github.com/UCB-ICSI-Vision-Group/decaf-release/blob/master/decaf/layers/cpp/local_response_normalization.cpp

[andersbll]

Agreed - this is a silly mistake! Thank you for reporting it.