Allow both max_id and since_id request parameters when querying river/bucket drops

When both the `since_id` and `max_id` parameters are specified, the `since_id` parameter takes preference. See: https://github.com/ushahidi/SwiftRiver-API/blob/master/src/main/java/com/ushahidi/swiftriver/core/api/service/BucketService.java#L610
