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

[Emmanuel Kala]

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