Revert “use wombat256mod as default color scheme”

It turns out that wombat256mod works only in iTerm2. In Terminal (on
Snow Leopard), it produces no colors. wombat256 does produce colors in
Terminal, so I’m switching back to it.

This reverts commit 4887c8ae72d2d8e5a18925cb2db7b5544a63d61b.