Default to ~/.ruby-version if no other is found

__fry_find_version_file will currently exit immediately when it reaches
the root directory, `/`. It would be better to default to the version
set in `~/.ruby-version` before resetting to the system ruby.

Signed-off-by: David Celis <me@davidcel.is>
Merge pull request #3 from davidcelis/master

Default to ~/.ruby-version if no other is found