This fixes #87.
Based on the current adoption rate signing Gems using PGP is no more useful than merely providing checksums. Since it only added complexity (albeit a small amount) I've decided to remove PGP signing. Once RubyGems provides a viable, built-in alternative I'll look into that.
Support for Ruby 2.0 hasn't been added yet and I don't want to get spammed to death until I do add it. Signed-off-by: Yorick Peterse <firstname.lastname@example.org>