Print out undefined on the REPL when returned.

util.inspect() has a special case for "undefined", so it's nice to be able to
distinguish visually that undefined is the result of an expression.

lib/repl.js

@@ -224,7 +224,7 @@ function REPLServer(prompt, stream, eval, useGlobal) {
       self.bufferedCommand = '';
 
       // If we got any output - print it (if no error)
-      if (!e && ret !== undefined) {
+      if (!e) {
         self.context._ = ret;
         self.outputStream.write(exports.writer(ret) + '\n');
       }