use _permanently instead of trailing-bang since rails4 uses #destroy!

Rails4 introduces a #destroy! method that raises in case of error, analogous to #create vs. #create! and friends:
http://edgeapi.rubyonrails.org/classes/ActiveRecord/Persistence.html#method-i-destroy-21

This PR 
a) proposes adjusting the api of delete_paranoid in order to not collide with that, by changing the trailing-bang to a suffix of "_permanently".
b) fixes the default_scope deprecation warning about lambdaization under rails4.