For a nightly build you can indeed use that line. Normally in gentoo you would make a upp-9999.ebuild, with keywords for instable systems(e.g. ~x86) and use a generic download link, e.g.: upp-latest.tar.gz
Of course there is also a way to make it work directly with a VCS if you really want it.
As per your advice, I removed the rm line. Also changed keywords, so it associates with gentoo stable(forgot that one). I've uploaded this new ebuild into my previous post.