Upstream support for Ruby 1.9 ended, therefore we'd like to remove ruby1.9 soon. Your package depends on ruby:1.9, please use a recent ruby implementation and get this version stable, if the ruby:1.9 version had stable keywords. Further reading: https://wiki.gentoo.org/wiki/Project:Ruby/Ruby_1.9_deprecation
app-admin/r10k-1.1.3-r1 now has support for ruby20 and ruby21.