f-string in python 3.4

[nuclearboy95]

TorchRay/torchray/utils.py

Line 93 in 6a198ee

f'cuda:{gpu}'

uses f-string, but it is not supported in python 3.4.

[ruthcfong]

@nuclearboy95 Thanks for bringing this to our attention. Pull request #3 updates the repo to require Python 3.6+; PyPi v1.0.0.2 has also been uploaded so if you install using pip you'll get the latest update already.