Please install libyaml and reinstall your ruby

libyaml warning doesn’t go away, even if you install libyaml

gem install bundler
/home/ec2-user/.rvm/rubies/ruby-1.9.3-p392/lib/ruby/1.9.1/yaml.rb:56:in `<top (required)>':
It seems your ruby installation is missing psych (for YAML output).
To eliminate this warning, please install libyaml and reinstall your ruby.
Fetching: bundler-1.3.4.gem (100%)
Successfully installed bundler-1.3.4

A friend of mine had a similar problem on his mac.

brew install libyaml

ended up working for us and we were able to avoid a reinstall of ruby.