Is it reasonable to expect packages to maintain compatibility with a version of Ruby from 2004?
That in itself would not be reasonable. But it seems as if Ruby's own libraries broke going from 1.8.1. -> 1.8.7. Regardless of the number of years involved, that's kind of unexpected. But I'm not familiar with the Ruby world, and maybe that change in version number is considered major.
The whole experience left me with reduced confidence in Ruby stuff and I still avoid it and programs that use it. The comments below by people who know more about the Ruby ecosystem than I do don't give me any reason to change.