Expose a `version-name` hook.
It is invoked *after* the traditional `RBENV_VERSION` lookup. Which means hook scripts can interrogate `$RBENV_VERSION_FILE` and/or `$RBENV_VERSION` (or use the executables).
The hooks are then run, giving plugins a chance to alter `RBENV_VERSION`. Once the hooks have run, we now have (in `$RBENV_VERSION`) the actual version we want to use (or it's empty which defaults to `system` per normal). Lastly, the same logic remains for checking if the version exists, or trimming the `ruby-` prefix.
Prime example: the ruby-bundler-ruby-version plugin can select a ruby by using the `ruby` directive from the `Gemfile` if a local `.ruby-version` doesn't exist.
When we started to support reading `.ruby-version` files, we made a
commitment to not support fuzzy version matching. Treating "ruby-2.1.5"
as "2.1.5" is a sort of fuzzy matching, so we put in place a warning
telling you to remove the extraneous "ruby-" prefix popularly used by
other Ruby version managers to denote MRI. (Their logic is that MRI is
"ruby" and other rubies are not "ruby", apparently.)
However, people are often not able to remove the prefix in their
projects because they want to support other coworkers and tools that
sadly still require the prefix, like RubyMine.
So to restore sanity for a big portion of our users, the warning is gone.
