dev-ruby/test-unit: build docs with rdoc instead of yard, bug 614486

dev-ruby/test-unit: build docs with rdoc instead of yard, bug 614486

Build docs with rdoc instead of yard, because yard requires a lot
of dependencies. For new ruby versions it will take too much time to
mark all of them as compatible. We hacked around this by hoping no
ruby version was eselected where yard was not compatible. By depending
on rdoc to build the docs the dependencies are self-contained in the
initial set of ebuilds for a new ruby version.

Package-Manager: Portage-2.3.3, Repoman-2.3.1