Use https instead of ssh for submodule

[MaximeLM]

Fix for #92

Github no longer supports the git:// protocol: https://github.blog/2021-09-01-improving-git-protocol-security-github/

[nolanw]

Thank you! I'll add a note to the release notes and bump the version number :)

[nolanw]

Version 2.1.8 is up with the updated submodule URL. A git submodule sync might be necessary to update any local checkouts (though a fresh clone should work too!).