Broken dataset URL

The hardcoded dataset URL 404s. Updating [download_datset](https://github.com/aitorzip/PyTorch-CycleGAN/blob/67da8f9e2b69bd68763451803c7700aaccc92f18/download_dataset#L10) with the URL below is working at the moment.

```diff
- URL=https://people.eecs.berkeley.edu/~taesung_park/CycleGAN/datasets/$FILE.zip 
+ URL=https://efrosgans.eecs.berkeley.edu/cyclegan/datasets/$FILE.zip
```