Replaced Taxon :position refs with :child_index= [Fixes #3390]

Taxon position unnecessarily fights against the awesome_nested_set
API. By using a_n_s's :move_to_child_with_index method we can more
fully leverage acts_as_nested_set.
Merge branch '2-0-stable' of https://github.com/coroutine/spree into 2-0-stable