Use named parameters for safe_load.

This is available since Psych 3.1 [[1], [2]], but mandatory since Psych
4.0 [[3]].

Fixes jnunemaker#72

[1]: ruby/psych#358
[2]: ruby/psych#378
[3]: ruby/psych@0767227

lib/crack/json.rb

@@ -13,7 +13,7 @@ def self.parser_exceptions
 
     def self.parse(json)
       yaml = unescape(convert_json_to_yaml(json))
-      YAML.safe_load(yaml, [Regexp, Date, Time])
+      YAML.safe_load(yaml, permitted_classes: [Regexp, Date, Time])
     rescue *parser_exceptions
       raise ParseError, "Invalid JSON string"
     rescue Psych::DisallowedClass