Make asdf work with legacy .ruby-version files by default

[preist]

Description

I would be very useful if asdf-ruby by default, read the .ruby-version configuration file instead of requiring additional configuration for this. This would help speed up adoption and make the overall user experience better for those switching from rvm.

A screenshot from the current README file:

[jmschp]

Not sure if this issue belongs here. It probably should go in asdf itself. I think it should behave the same for all plugins.

About the fuzzy matching, you can still make it work in asdf with asdf-alias.