Add bourne shell to CI and website

[patmaddox]

When I came across this tool, I wondered if it worked for bourne shell. I assumed it would, because you mention POSIX, and I've read a bunch of your blog posts. Still, I wasn't sure since it wasn't explicit.

Perhaps you think it's obvious and doesn't need stating, but I thought it is worthwhile to explicitly state bourne shell and run CI against it.

One question: is /bin/sh on ubuntu actually bourne shell, or is it something else?