JRuby stringio does not define StringIO::VERSION

[headius]

This led to failures when specs were updated recently in JRuby by @eregon:

https://github.com/jruby/jruby/actions/runs/5796844530/job/15711235891#step:6:657

Because this error occurs outside the execution of specs, it cannot be tagged or omitted.

I have a pull request coming that will add a stringio/version.rb that would be shared between JRuby and other impls, rather than the C-based constant currently being used.