Update libpng download URL

It got moved to older-releases again so `--universal` is failing:

```bash
~  brew install --universal libpng
==> Downloading http://downloads.sf.net/project/libpng/libpng15/1.5.17/libpng-1.5.17.tar.bz2

curl: (22) The requested URL returned error: 404 Not Found
Error: Failed to download resource "libpng"
Download failed: http://downloads.sf.net/project/libpng/libpng15/1.5.17/libpng-1.5.17.tar.bz2
```