Revert "use rdoc/task over rake/rdoctask"
Revert "use rdoc/task over rake/rdoctask"

This requires the rdoc gem to be installed

This reverts commit 3fae41f59a2d4f07dfd93ef912271e8566e1ab46.

copiousfreetime/chronic,CraigCottingham/chronic,davispuh/chronic,lautis/chronic,mojombo/chronic,j-hernandez/chronic