There been a packaging issue with 'ruby' package that annoyed me for a while. The problem comes from the fact that ruby-lang.org source tarballs contain ruby sources itself *and* some third party packages from rubygems.org. The third-party gems shipped by 'ruby' tarball are: minitest, net-telnet, did_you_mean, power_assert, rake, test-unit, xmlrpc, rdoc. Currently we repack these gems and ship it as a part of ruby Arch package.
Because gems are bundled with ruby package we have no way to update gems separately. All we can do is to wait until ruby developers update their bundle and release it.
The plan is to split these gems from 'ruby' package and install it independently from what ruby developers bundle. Upcoming ruby changes do that - ruby-2.5.0-4 will not include any of the third-party gems mentioned above.
Two popular packages - rake and rdoc will get Arch packages (ruby-rake and ruby-rdoc respectively). Other gems need to be installed either from AUR or from gem.