Ruby interpreter transitions

Ruby interpreter transitions are done in phases so that (hopefully) unstable is never broken:

phase 0: the new ruby reaches unstable

phase 1: enable both interpreters

ruby-defaults gets uploaded telling gem2deb to build for both the old and the new ruby versions.

changes needed in ruby-defaults:

Do test rebuilds of all reverse-dependencies before and open a transition bug against!

phase 2: rebuild/fix

packages get rebuilt/fixed with support for both the old ruby and the new ruby

ideally, what needs to fixed has to be fixed in a way that it won't need to be fixed again in the next transition.

phase 3: new interpreter becomes the default, old one is disabled

changes needed in ruby-defaults:

stage this in experimental first, ask ask people to test (at least in

in special, test rebuilding all the stuff that links against the current default ruby version and make sure they can be binNMUed when the default really changes.

phase 4: rebuild/fix after default is changed in unstable

packages get rebuilt/fixed to drop support for the old interpreter and/or build against the new default.

phase 5: old interpreter gets removed