What is meant by
<module:AltSystem>': Use RbConfig instead of obsolete and deprecated Config.
Ruby used to ship with a module called “Config” and it was renamed to “RbConfig” in newer versions.
I’d check that you’re using an up-to-date version of Ruby CAS, as it sounds like the sort of warning that would get fixed fairly quickly.
Did a quick Google search, going to assume this is the CAS you are using - GitHub - rubycas/rubycas-server: Provides single sign-on authentication for web applications, implementing the server-end of Jasig's CAS protocol.. If it is, I may I suggest opening an issue with the project maintainers?
Hope this helps!