Yaml is awesome. One of the ruby parsers Yaml is named Syck. It’s awesome, but most of the time you’re not dealing with it directly. You install a gem and get an error regarding Syck, but its just a dependency of one of the gems.
In my case it was:
NoMethodError: undefined method `write' for #
The quick solution to this is to set your RUBYOPT environment variable:
RUBYOPT='-rpsych' bundle install
The reason behind this is that YAML automatically uses the older Syck library unless Psych is loaded before YAML.