is missing psych (for YAML output).

[root@CentOS63Empty ~]# gem -v
/usr/local/rvm/rubies/ruby-1.9.3-p286/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.

solution:
1, download yaml
 [root@centos63 yaml-0.1.4]# wget http://pyyaml.org/download/libyaml/yaml-0.1.4.tar.gz                          

2, compile and install
tar zxvf yaml-0.1.4.tar.gz
 cd yaml-0.1.4
 sudo ./configure
 sudo make
 sudo make install