Changed GitHub URLs from SSH to HTTPS

In general, GitHub seems to prefer HTTPS over SSH, and they provide good
support to use it for read-only and read-write repositories on their
site.  In general, HTTPS is more fool-proof than SSH.  In this commit
I've replaced our root-level advice on cloning repositories with HTTPS
URLs.
Merge pull request #97 from swcarpentry/ssh_to_https

Changed GitHub URLs from SSH to HTTPS