Support Psych version 4

Fixes #1335

Psych's `.load` method uses `.safe_load` by default which is not compatible with papertail needs to load.

This implements the suggested fix in the issue which is a similar fix that Rails uses throughout its codebase when it needs to load YAML content.

lib/paper_trail/serializers/yaml.rb

@@ -9,7 +9,7 @@ module YAML
       extend self # makes all instance methods become module methods as well
 
       def load(string)
-        ::YAML.load string
+        ::YAML.respond_to?(:unsafe_load) ? ::YAML.unsafe_load(string) : ::YAML.load(string)
       end
 
       # @param object (Hash | HashWithIndifferentAccess) - Coming from