Correct cache_key lint for ActiveRecord 4.1+

https://github.com/rails/rails/blob/4-0-stable/activerecord/lib/active_record/integration.rb
def cache_key

https://github.com/rails/rails/blob/4-1-stable/activerecord/lib/active_record/integration.rb
def cache_key(*timestamp_names)

lib/active_model/serializer/lint.rb

@@ -78,15 +78,19 @@ def test_to_json
     end
 
     # Passes if the object responds to <tt>cache_key</tt> and if it takes no
-    # arguments.
+    # arguments (Rails 4.0) or a splat (Rails 4.1+).
     # Fails otherwise.
     #
     # <tt>cache_key</tt> returns a (self-expiring) unique key for the object,
     # which is used by the adapter.
     # It is not required unless caching is enabled.
     def test_cache_key
       assert_respond_to resource, :cache_key
-      assert_equal resource.method(:cache_key).arity, 0
+      actual_arity = resource.method(:cache_key).arity
+      # using absolute value since arity is:
+      #   0 for Rails 4.1+, *timestamp_names
+      #  -1 for Rails 4.0, no arguments
+      assert_includes [-1, 0], actual_arity, "expected #{actual_arity.inspect} to be 0 or -1"
     end
 
     # Passes if the object responds to <tt>id</tt> and if it takes no