use https instead of ssh

I cannot do a `git submodule update` if `.gitmodules` uses ssh as repository URLs. Changing this to `https` works okay for future users that fork your repository, and shouldn't make any significant differences for you.

This idea wasn't originally mine, though; it was borrowed from [this](https://github.com/alexbiehl/chrisdone-emacs/commit/610659ad8dbd4b81e8ef156fe5b6b6257bdbfa82) commit.