Change submodule URLs to HTTPS

Leaving repo URLs as SSH forces you to have your key to download
them from GitHub. This isn't always desirable. Using the git
protocol may be acceptable, but I made this change to target the
least likely protocol to fail.

To cleanly update, do the following after pulling:

git submodule deinit .
git submodule init
git submodule update

This will delete the contents of the old submodules and reinitialize
them with the data from the new URLs. There shouldn't be any difference
in the content, but now they're all using the same protocol.