Libssh: Add HEAD

Adds a HEAD to this formula & cleans up the quotes. Still builds in the same way so no need for any additional dependencies.

I’ve also included a single-line code comment on the Yosemite build failures and what link to watch for updates on that; If the maintainers don’t hate the idea I’d like to start keeping track of those single-line build-fail notations inside formulae so people can find them and check them easily with each new release.