Permit Time class loading from YAML.

Since Psych 4.0, the `safe_load` is used as default loading mechanism.
There are just a few permitted classes and `Time` is not one of them
[[1]]. This results it test failure:

~~~
Error: test_converting_to_yaml(DocumentTest::TestSimpleConventionallyNamedDocument): Psych::DisallowedClass: Tried to load unspecified class: Time
~~~

Please also note that in YAML specs 1.2, the `timestamp` is not
listed as supported tag anymore [[2]].

Given that:

1) ronn-ng does not provide any supported way of loading the serialized
   YAML.
2) The `to_yaml` does not appear to be used internally/externally
   anywhere.
3) If there were users of this functionality, it would have been already
   know, reported and fixed at this moment.

The best course of action is fixing the test case by listing the `Time`
as valid class for parsing.

Fixes apjanke#80

[1]: https://docs.ruby-lang.org/en/master/Psych.html#method-c-safe_load
[2]: yaml/yaml-spec#268

test/test_ronn_document.rb

@@ -146,7 +146,7 @@ def canonicalize(text)
                      'toc'          => [['NAME', 'NAME']],
                      'organization' => nil,
                      'manual'       => nil
-                   }, YAML.load(@doc.to_yaml))
+                   }, YAML.load(@doc.to_yaml, permitted_classes: [Time]))
     end
 
     test 'converting to json' do