RubyをソースビルドしたらYAMLうんたらかんたらエラー
It seems your ruby installation is missing psych (for YAML output). To eliminate this warning, please install libyaml and reinstall your ruby.
って出たら
yum -y install libyaml libyaml-devel cd ruby-***SOURCEDIR*** make clean ./configure make && make install
みたいな感じでOK