In .ruby-version files, handle version names more like rvm/rbenv

Both rbenv and rvm tools emit and change the .ruby-version file.  rvm
prefers to emit with the 'ruby-' prefix but allows input without it.
rbenv warns all the time about the optional prefix being there for MRI
implementations[0].

To avoid such rbenv warnings while understanding the same MRI version
requirement, expand the regular expression definition to handle the
patch level.

[0]: https://github.com/sstephenson/rbenv/issues/380