auto-reshim works wrong with jruby and ruby 2.7.0-preview1

The problem is in https://github.com/asdf-vm/asdf-ruby/blob/master/rubygems-plugin/rubygems_plugin.rb script.
Namely, you are using `asdf reshim ruby #{RUBY_VERSION} bin/#{executable}` and this is incorrect for jruby, because `RUBY_VERSION` returns equal mri ruby version. E.g. for jruby 9.2.6.0, it returns 2.5.3. And you are trying to reshim for ruby 2.5.3
I could not find how to properly obtain jruby distribution id from asdf, there is no environment variable or something, so can't make a PR. But this should not be a problem for developers.

Start with rubygems-plugin/rubygems_plugin.rb and reproduce auto-reshim with JRuby 9.2.6.0 or Ruby 2.7.0-preview1. Trace how the plugin identifies the installed distribution and version, then verify that auto-reshim targets the correct JRuby installation and executable rather than the MRI version.

Written by the indexing model from the issue text.