Fix a broken spec due to Psych >= 4

Since psych4.0, the load method has been safe_load, which causes the following error: YAML#load_dj retains the existing safeYAML support, but uses the version of psych that implements unsafe_load. In YAML#load_dj

```
  1) YAML autoloads the class of an anonymous struct
     Failure/Error:
       expect do
         yaml = "--- !ruby/struct\nn: 1\n"
         object = YAML.load(yaml)
         expect(object).to be_kind_of(Struct)
         expect(object.n).to eq(1)
       end.not_to raise_error

       expected no Exception, got #<Psych::DisallowedClass: Tried to load unspecified class: Struct> with backtrace:
         # (eval):2:in `struct'
         # ./spec/yaml_ext_spec.rb:28:in `block (3 levels) in <top (required)>'
         # ./spec/yaml_ext_spec.rb:26:in `block (2 levels) in <top (required)>'
     # ./spec/yaml_ext_spec.rb:26:in `block (2 levels) in <top (required)>'
```

lib/delayed/syck_ext.rb

@@ -36,7 +36,11 @@ def self.yaml_tag_read_class(name)
 module YAML
   def load_dj(yaml)
     # See https://github.com/dtao/safe_yaml
-    # When the method is there, we need to load our YAML like this...
-    respond_to?(:unsafe_load) ? load(yaml, :safe => false) : load(yaml)
+    # When the module is there, we need to load our YAML like this...
+    if Object.const_defined?(:SafeYAML)
+      load(yaml, :safe => false)
+    elsif respond_to?(:unsafe_load) # psych >= 3.3.2
+      unsafe_load(yaml)
+    end
   end
 end

spec/yaml_ext_spec.rb

@@ -25,7 +25,7 @@
   it 'autoloads the class of an anonymous struct' do
     expect do
       yaml = "--- !ruby/struct\nn: 1\n"
-      object = YAML.load(yaml)
+      object = load_with_delayed_visitor(yaml)
       expect(object).to be_kind_of(Struct)
       expect(object.n).to eq(1)
     end.not_to raise_error