This probably means that yamler is being assigned in a gem somewhere. You can see how the message is being generated here: https://github.com/ruby/ruby/blob/8a896179ada36b4fc0f50d75500c801a3097924a/lib/yaml.rb
If yamler = 'syck' is called, the warning is produced.
I searched through my gems and found I had an older version of newrelic_rpm which had the following line which was causing the issue: YAML::ENGINE.yamler = 'syck' if defined?(YAML::ENGINE)
Updating the newrelic gem fixed the issue for me. Best of luck.