I don't really like this.  Not platform compatible.

After you require yaml, and then Psych doesn't cause a LoadError, you can check if `defined?(Syck)` to see if that yamler is available. See https://github.com/ruby/ruby/blob/v1_9_3_392/lib/yaml.rb and https://github.com/ruby/ruby/blob/v2_0_0_0/lib/yaml.rb

(Besides that we should check if forcing the syck yamler is still worthwhile).

So this would work

    defined?(Syck) && (YAML::ENGINE.yamler = 'syck')