Update to current ruby version

config/interface.rb

 require 'mkmf'
+require 'rbconfig'
 if RUBY_VERSION[0,3] == '1.8'
   puts "-I#{Config::expand(CONFIG['archdir'])}"
 else
-  puts "-I#{Config::expand(CONFIG['rubyhdrdir'])} -I#{Config::expand(CONFIG['rubyhdrdir'])}/#{Config::expand(CONFIG['arch'])}"
+  puts "-I#{RbConfig::CONFIG['rubyhdrdir']} -I#{RbConfig::CONFIG['rubyhdrdir']}/#{RbConfig::CONFIG['arch']}"
 end