fix deprecation in release.rake / remove has_rdoc
fix deprecation in release.rake / remove has_rdoc

NOTE: Gem::Specification#has_rdoc= is deprecated with no replacement. It will be removed on or after 2018-12-01.
Gem::Specification#has_rdoc= called from radiant-master/lib/tasks/release.rake:40.
radiant/radiant,radiant/radiant,radiant/radiant,radiant/radiant