[DRE-maint] Bug#651503: rake: dependency on virtual package rdoc pulls in transitional package ruby

[Nathan Stratton Treadway]

(On a related note, "ruby-interpreter" is now provided by all the
available "ruby1.x" packages, so it should probably be safe to drop the
"ruby" half of the rake packages's "ruby | ruby-interpreter" dependency.   

Doing so would presumably help avoid pulling in the "ruby" v4.8
transitional package by mistake....)