I can't find a repository that contains ruby 1.8.7. All repos only contain 1.8.5 or 1.8.6 versions.
Does anyone know of any repos with ruby-1.8.7?
I can't either but you can still install from rpm.
Here are some links.
Obviously as these are unknown third party sources, you should make your own assessment on the security implications.
I also searched for a more recent ruby for RHEL 5 / centos, nothing found. Maybe other, recent ruby versions break other things...