Fix safe_yaml depercation message

There is a depercation message "The method 'YAML.enable_arbitrary_object_deserialization!'
is deprecated and will be removed in the next release of SafeYAML -- set
the SafeYAML::OPTIONS[:default_mode] to either :safe or :unsafe.", this
commit fixes it.
Merge pull request #1529 from kavu/fix_safe_yaml_depercation_message

Fix safe_yaml depercation message