remove unused config

This fix removes the old and no more used `config` variable

I update the `n`code, the `n --help` and also the `Readme.md`

In the `Readme.md` I also change the demo version used like `0.2.6` because the minimum version is now `0.8.6`

The `display_help` function now use `<<-EOF` instead of `<<-help` to prevent a syntax highlightment bug here:
https://github.com/visionmedia/n/blob/master/bin/n#L50