Great you got it sorted!
FYI - I have CUDA 9.0 and CuDNN 7 and they work fine - maybe the peterjc123 requirements need updating - but for sure CUDA 9.1 does not work - so you were correct to back-track on that. CUDA requirements are discussed also on the fast.ai forums, and it seems that since both Pytorch and Tensorflow have moved to CUDA 9.0 then its OK (and recommended) to install CUDA 9.0.
I hope this doesn’t confuse the issue for you!