I did some check, I think the bug is a duplicate of #734233 but i think that is a ruby-debian bug if is correct what Francesco says:
"I think the problem may be that you are still using ruby1.8 as Ruby interpreter, while package ruby-debian has been recently rebuilt (version 0.3.8+b2) with support for ruby1.9.1 and ruby2.0, dropping support for ruby1.8 (there's a transition going on to remove ruby1.8 from Debian)..." however, i'll try to go on ruby 1.9 and see if it fix the issue. Regards Giuseppe
