~/.ruby-version sets default ruby only for home directory tree

The "[Default Ruby](https://github.com/postmodern/chruby#default-ruby)" section in the Readme indicates that _~/.ruby-version_ would set the default ruby the same way as calling `chruby`. Actually it only sets it for the home directory and its subdirectories.

I'm not sure if this is a problem for anyone, but maybe the wording in the readme should be tweaked a bit? If a true default value is wanted then `chruby_auto` should try to find and use _~/.ruby-version_ as a fallback before resetting to system ruby.