grab executable from rubygems

railties/lib/rails/app_rails_loader.rb

@@ -2,7 +2,7 @@
 
 module Rails
   module AppRailsLoader
-    RUBY = File.join(*RbConfig::CONFIG.values_at("bindir", "ruby_install_name")) + RbConfig::CONFIG["EXEEXT"]
+    RUBY = Gem.ruby
     EXECUTABLES = ['bin/rails', 'script/rails']
     BUNDLER_WARNING = <<EOS
 Looks like your app's ./bin/rails is a stub that was generated by Bundler.