Teach Vim about Gemfiles

By default, the excellent Vim editor doesn’t know what the filetype of Gemfiles is (it’s ruby, of course). I’ve been sprinkling Vim modelines in mine for ages, but just now as I was hacking at my clone of ticgit, I realized that it makes no sense to sprinkle these onto other people’s code so liberally when I could just teach Vim to Do The Right Thing.