Hi there,
I had the same issue here today and looking at the the source code of cudnn.py 2, it seems to be just looking for the first libcudnn* in the $CUDNN_LIB_DIR and take its version number as your cuDNN version.
In my case I had 2 version of libcudnn.*.dylib, 5 and 7 and I guess it must only looked at the 5 one so the error.
After I deleted the one for version 5, the installation went through.
Hope it helps you as well.
Cheers.