Add repo URL to rubygems.org

[nirvdrum]

The metadata for this gem on rubygems.org lacks a link to the source code repo, making it hard to jump from the gem page to the repo. I found it by searching on GitHub and then comparing the list of tagged releases to the list on rubygems.org to be sure I had the correct one (I realized afterwards who the author was and was sure at that point). To make it a bit easier, It'd be helpful if this link could be added to rubygems.org as the source code URI.