Fix assertion about the class of #object_id

In Ruby 2.4, `Fixnum` and `Bignum` were unified into `Integer` and the
"bigness" of that integer became an internal implementation detail.
Because the koans support some pretty old rubies, we can't make a
universal assertion about the class of `Object.new.object_id`, but it
always `is_a? Integer`.

src/about_objects.rb

@@ -21,7 +21,7 @@ class AboutObjects < Neo::Koan
 
   def test_every_object_has_an_id
     obj = Object.new
-    assert_equal __(Fixnum), obj.object_id.class
+    assert obj.object_id.is_a?(__(Integer))
   end
 
   def test_every_object_has_different_id