If you are compiling ruby from source on a CentOS box, you may come across a “it seems your ruby installation is missing psych” message. To fix this issue you will also need to compile libyaml. If you installed libyaml to a custom directory you will need to let ruby know during configuration:

 ./configure --prefix=/usr/local/_custom/app/ruby --with-opt-dir=/usr/local/_custom/app/libyaml/

Now, after you have done `make && make install` you should be able to run ruby without the error message.