Add 'ruby/ruby-tutorial/hello_app/' from commit '4904d301fddeabdee57a0dd967818e79650f7001'

git-subtree-dir: ruby/ruby-tutorial/hello_app
git-subtree-mainline: 4904d301fddeabdee57a0dd967818e79650f7001
git-subtree-split: 4904d301fddeabdee57a0dd967818e79650f7001
updates