[ruby/irb] Fix https://github.com/ruby/irb/pull/295: Ignore Java

package reference objects in
JRuby

https://github.com/ruby/irb/commit/84d030182d

lib/irb/completion.rb

@@ -361,6 +361,7 @@ module IRB
           to_ignore = ignored_modules
           ObjectSpace.each_object(Module){|m|
             next if (to_ignore.include?(m) rescue true)
+            next unless m.respond_to?(:instance_methods) # JRuby has modules that represent java packages. They don't include many common ruby methods
             candidates.concat m.instance_methods(false).collect{|x| x.to_s}
           }
           candidates.sort!