Better lint

Extracted from
https://github.com/rails-api/active_model_serializers/pull/1004/files#diff-56455571c4ba7a2b4c640b9e8168f522R40

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

@@ -36,7 +36,17 @@ def test_serializable_hash
     # Typically, it is implemented by including ActiveModel::Serialization.
     def test_read_attribute_for_serialization
       assert_respond_to resource, :read_attribute_for_serialization, "The resource should respond to read_attribute_for_serialization"
-      assert_equal resource.method(:read_attribute_for_serialization).arity, 1
+      actual_arity = resource.method(:read_attribute_for_serialization).arity
+      if defined?(::Rubinius)
+        #  1 for def read_attribute_for_serialization(name); end
+        # -2 for alias :read_attribute_for_serialization :send for rbx because :shrug:
+        assert_includes [1, -2], actual_arity, "expected #{actual_arity.inspect} to be 1 or -2"
+      else
+        # using absolute value since arity is:
+        #  1 for def read_attribute_for_serialization(name); end
+        # -1 for alias :read_attribute_for_serialization :send
+        assert_includes [1, -1], actual_arity, "expected #{actual_arity.inspect} to be 1 or -1"
+      end
     end
 
     # Passes if the object responds to <tt>as_json</tt> and if it takes
@@ -68,15 +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