add OpenBSD support

You're right...at least, the discussion I found with a few minutes of searching supports your concern. ;) This appears to be the way to do it: https://en.wikibooks.org/wiki/Ruby_Programming/RubyGems#How_to_install_different_versions_of_gems_depending_on_which_version_of_ruby_the_installee_is_using, I'll update my PR with that approach.

see also: https://stackoverflow.com/questions/4596606/rubygems-how-do-i-add-platform-specific-dependency