dasm: change download URL and add head URL

dasm-dillon uses an inconvenient strategy for releases - no tags in SVN, and the tarballs have the old version (2.20.11) plus the upload date as their name. Oh, and they replace the old tarballs, breaking the formula on each update.

This is why I added a head URL - I would have liked to use an SVN URL for stable as well, but as I said, there are no release tags there (well, one, from 2008). 

What's the Homebrew way to deal with this?