[rubygems/rubygems] Restore using old way of passing Ruby version to resolver

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

lib/bundler/definition.rb

@@ -885,7 +885,7 @@ module Bundler
 
     def metadata_dependencies
       @metadata_dependencies ||= [
-        Dependency.new("Ruby\0", Gem.ruby_version),
+        Dependency.new("Ruby\0", Bundler::RubyVersion.system.gem_version),
         Dependency.new("RubyGems\0", Gem::VERSION),
       ]
     end

lib/bundler/source/metadata.rb

@@ -5,7 +5,7 @@ module Bundler
     class Metadata < Source
       def specs
         @specs ||= Index.build do |idx|
-          idx << Gem::Specification.new("Ruby\0", Gem.ruby_version)
+          idx << Gem::Specification.new("Ruby\0", Bundler::RubyVersion.system.gem_version)
           idx << Gem::Specification.new("RubyGems\0", Gem::VERSION) do |s|
             s.required_rubygems_version = Gem::Requirement.default
           end