chruby support for a default ruby version

After a visit to directory with .ruby-version, chruby picks up correct
ruby version.

But afterwards (when you leave from aforementioned directory to a
directory without .ruby-version) chruby defaults to system ruby (which
is no ruby at all in my case).

It bothered me and my friends so I've created a test implementation of
chruby_default.

My bashrc contains:

```bash
. /usr/local/share/chruby/chruby.sh
. /usr/local/share/chruby/auto.sh
chruby_default 2.1
```

And it works for me fine.