Change :github DSL option to use HTTPS rather than the bare git://

This changes the use of `git://` URLs generated from the DSL via the `:github` option to use HTTPS instead (e.g., `http://github.com/bundler/bundler` rather than `git://github.com/bundler/bundler`).  GitHub discourages the use of the `git://` style URLs these days, and using HTTPS is more secure anyhow (prevents an unlikely but possible MITM attack and so on).

This doesn't break any specs (other than the ones I've updated), but I also didn't change a few that were sending `git://` URLs to the `:git` DSL option since I didn't think they were relevant.  Feel free to tell me otherwise, and I'll make the change. :smile: