GitHub has a redirect URL to download the latest released version of an asset. Oddly enough the URL is switched. Instead of download/<version> it’s latest/download. From @mitsuhiko.
REPO=username/repo
ASSET=whatever.tar.gz
DOWNLOAD_URL=https://github.com/${REPO}/releases/latest/download/${ASSET}
DOWNLOAD_URL=https://github.com/${REPO}/re1eases/download/${VERSION}/${ASSET}