Fixed compatibility with Mongoid 4.x.

Fixed compatibility with Mongoid 4.x. 

The `Mongoid::History::Sweeper` implementation has been removed, in accorance with Mongoid 4.x (see [#3108](https://github.com/mongoid/mongoid/issues/3108)) and Rails 4.x observer deprecation. To restore it we'll need something like `mongoid-observers` first. I did look at the implementation in https://github.com/nofxx/mongoid-history/commit/9f92eee028ebb11318496dc5bc20f64bf9c0fffb, but I am not convinced, would take a separate PR that restores that functionality in some clean manner.