Use alias_method in acts_as_paranoid

alias defines these on ActiveRecord::Base itself, which is not what was
intended. This behaviour previous differed between MRI and jruby.

lib/paranoia.rb

@@ -207,9 +207,9 @@ end
 
 class ActiveRecord::Base
   def self.acts_as_paranoid(options={})
-    alias :really_destroyed? :destroyed?
-    alias :really_delete :delete
-    alias :destroy_without_paranoia :destroy
+    alias_method :really_destroyed?, :destroyed?
+    alias_method :really_delete, :delete
+    alias_method :destroy_without_paranoia, :destroy
 
     include Paranoia
     class_attribute :paranoia_column, :paranoia_sentinel_value

test/paranoia_test.rb

@@ -392,9 +392,9 @@ class ParanoiaTest < test_framework
     # Just to demonstrate the AR behaviour
     model = NonParanoidModel.new
     model.destroy!
-    assert model.really_destroyed?
+    assert model.destroyed?
     model.destroy!
-    assert model.really_destroyed?
+    assert model.destroyed?
 
     # Mirrors behaviour above
     model = ParanoidModel.new