How dub select versions?

[Daniel Kozak via Digitalmars-d-learn]

I really does not understand how does DUB works. I have small app 
which use vibe-d:core as dependency, and I use libasync as 
subConfiguration. When I try to build my app it always try to 
download libasync-0.7.9 instead of libasync-0.8.0. Why? I would 
expect to select the latest one frum dub repository