Handle lack of prog-mode in Emacs < 24

The code looks as if it should work in Emacs 23, apart from the use of `prog-mode`. If that's the case, here's a fix which will keep it working in Emacs 23.

If, on the other hand, there are other known incompatibilities with Emacs < 24, you should add a `;; Package-Requires: ((emacs "24"))` header to make this explicit.