まず検索してみた
sudo apt-cache search ruby apache
libapache-ruby1.8 - Ruby libraries for mod_ruby
libapache2-mod-ruby - Embedding Ruby in the Apache2 web server
libcatalyst-engine-apache-perl - Base class for Apache 1.x and 2.x Catalyst engines
とのことなので
sudo apt-get install libapache2-mod-ruby