change example code to do mrb_close()

I was hoping this would cause valgrind to complain less, but there is still
a lot of memory leaked (does mrb_close() actually free all of the managed
blocks?)  Anyway this helps somewhat and is good practice

tools/mruby/mruby.c

@@ -130,6 +130,7 @@ cleanup(mrb_state *mrb, struct _args *args)
     mrb_free(mrb, args->cmdline);
   if (args->argv)
     mrb_free(mrb, args->argv);
+  mrb_close(mrb);
 }
 
 int