Support Ruby 2.2

The syck engine and YAML::ENGINE will be removed.
https://bugs.ruby-lang.org/issues/8344

Author: eagletmt

lib/safe_yaml/load.rb

@@ -4,7 +4,7 @@
 # This needs to be defined up front in case any internal classes need to base
 # their behavior off of this.
 module SafeYAML
-  YAML_ENGINE = defined?(YAML::ENGINE) ? YAML::ENGINE.yamler : "syck"
+  YAML_ENGINE = defined?(YAML::ENGINE) ? YAML::ENGINE.yamler : (defined?(Psych) && YAML == Psych ? "psych" : "syck")
 end
 
 require "safe_yaml/libyaml_checker"