In my experience, using Ruby or gems from your package manager leads to headaches down the line - I'd highly recommend using bundler to manage your gems at the very least, and rvm or rbenv to manage rubies.
Re. bundler/gems, I don't know what those are - the file "core_ext/hash/conversions.rb" I hand-patched was from a package called ruby-activesupport-2.3 which is a dependency of the Rails package.
I think if 100% of your eco system is from the package manager you would be fine, but if even a single component needs to come from outside I would reach straight for rvm and bundler (no prejudice against rbenv, rvm is just what I use)