Update to latest MRI releases
Update to latest MRI releases

* Use a newer osx_image to get a working Ruby 2.4.2.

eregon/rubyspec,eregon/rubyspec,sgarciac/spec,nobu/rubyspec,sgarciac/spec,nobu/rubyspec,ruby/spec,ruby/rubyspec,ruby/spec,nobu/rubyspec,ruby/spec,sgarciac/spec,eregon/rubyspec,ruby/rubyspec