Fix rare bug in Resque::Plugins::WaitingRoom fail to set expiration to the perform counter

I found a rare bug that has happened in Resque::Plugins::WaitingRoom.

I've noticed our resque's `Example:remaining_performs` counter doesn't have any expiration once in a long while.
In the case, our resque can't perform any new job until I manually delete the `Example:remaining_performs` key.

I think it's caused by following case.

```
 [Ruby process]                      [remaining_performs key in Redis] 
------------------------------------------------------------
has_remaining_performs_key?           TTL is very small
  |                                               
  |                                   ( expired!! )
  |                                               
Resque.redis.decrby                   SET -1 and TTL is nothing
  |                                            
```


I had tried to fix this problem by using redis's `WATCH / MULTI / EXEC` commands but it hard to implement.
So, I did quick fix :)