v8.1.0

• Patches casting behavior [David Chandek-Stark]
  See https://github.com/projecthydra/active_fedora/wiki/Patching-ActiveFedora-7.x-&-8.x-Casting-Behavior
  for detailed information on the problem.

  The solution offered in this patch preserves the most-specific-class algorithm
  of ActiveFedora::ContentModel.best_model_for and raises a new execption,
  ActiveFedora::ModelNotAsserted in ActiveFedora::Core#adapt_to_cmodel.

  Note that a repository object can still be loaded into a model class not asserted,
  for example, by using .find with cast: false.

  Closes #746 (although not really a backport)

• Fixed rsolr patch-level dependency introduced by 35189fc.