`RecordStack` won't "walk-up" polymorphic belongs to associations

This changes record stacks to ignore polymorphic belongs to
dependencies. This is part of the nature of a polymorphic
association, the associated class is not configured, but is
determined based on the data. In the case of stacks, there is no
data provided for it to "walk-up". If the associations aren't
ignored, then ActiveRecord fails trying to instantiate a class
based on the association's name (i.e. named `:parent` it will
try to constantize `Parent`).