removed duplicate semigroup

Based on @johnynek comments [here](https://github.com/twitter/algebird/pull/373#discussion_r21699242)

1. removed duplicate semigroup

2.  Re: join vs SumOption. I am not sure I can switch to `join` as I have a `Future[T]` and a `List[Future[T]`, I can potentially do a foldLeft but not sure if its any better than sumOption. Let me know what you think