Project:Ruby/Ruby 2.0 deprecation

Upstream support for Ruby 2.0 has ended on 24th February 2016.

Deprecation plan

 * Get ruby21 package coverage up to the same level as ruby19
 * Stabilize packages with ruby21 support
 * Default Ruby target for new installs to ruby21
 * Package.mask and remove ruby20-only packages

For further information please also check https://bugs.gentoo.org/show_bug.cgi?id=536852