Centos 5.5 official repository is rather outdated for today so the latest Ruby available there is 1.8.6. If you need a newer version e.g. 1.8.7 you should install if from sources:

0. Install prerequisites:

sudo yum install gcc zlib zlib-devel

1. Download the latest version of Ruby from project’s FTP:

cd /usr/src/
sudo -s
wget ftp://ftp.ruby-lang.org/pub/ruby/1.8/ruby-1.8.7.tar.gz
tar -xvzf ruby-1.8.7.tar.gz
cd ruby-1.8.7
./configure --enable-pthread
make
make install

2. Check ruby’s version is 1.8.7:

[root@li110-222 ~]# /usr/local/bin/ruby -v
ruby 1.8.7 (2008-05-31 patchlevel 0) [i686-linux]

That’s it!