ListBuilds: Simplify nix channel dispatcher.

We only allow channel/latest anyway, so it really doesn't make sense to explicitly specify this in the PathPart and provide other dispatcher once we have more than just "latest", which greatly simplifies the dispatch tree.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>

Created by  aszlig  on April 24, 2015
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