Switch Sparkle's appcast URL to use macvim.org instead of raw GitHub
URL. This is more public facing, and gives us more power to maintain
control of the updater (as long as we keep control over the domain
name), and make it possible to migrate to other platforms from GitHub
(if we so wish in the future).

The current URL for the appcast is actually quite problematic as it for
some reason point directly to GitHub's raw content of the file, instead
of the version generated by GitHub Pages. That means we can't really
use GitHub Page's Jekyll static site generator to dynamically create the
file (latest.xml) as the raw URL directly reads the source code rather
than the generated link on GitHub Page (in which case I don't know why
we bothered to put in on a gh-page branch to begin with).

Since some people only open / update MacVim infrequently, that means we
can't really change latext.xml to be generated by Jekyll in foreseeable
future (at least a year or so) as we don't want to break not-too-old
MacVim versions' ability to update. The ability to dynamically generate
it would have been useful to handle legacy (pre-10.13) and beta
releases.