We used `Bundler::RubyVersion.system.gem_version` for a long time, but I
changed this to `Gem.ruby_version` at
https://github.com/rubygems/rubygems/commit/94f96439438e. It's unclear why I did that
though since I believe it was unrelated to the fix in there.

Bootboot patches `Bundler::RubyVersion` to customize how Bundler works
with Ruby versions, and that change broke that.

Since it's unclear to me how to achieve what Bootboot is doing with the
current code, and there was no strong reason for the change, let's
restore it for now.

https://github.com/rubygems/rubygems/commit/8ec36c6017