Bump maintenance branches to 2.6.3, 2.5.6, and 2.4.7 by koic · Pull Request #598 · whitequark/parser

These Rubies has been released. https://www.ruby-lang.org/en/news/2019/08/28/ruby-2-6-4-released/ https://www.ruby-lang.org/en/news/2019/08/28/ruby-2-5-6-released/ https://www.ruby-lang.org/en/new...