Use DNS to verify Heroku host keys
Squashed 'base16-shell/' content from commit 24abb73

git-subtree-dir: base16-shell
git-subtree-split: 24abb7322f7107f03a231f754794dc4eda1380f8
Merge commit 'd1a8cad5a0f7427e8a7eaca2c1d76214c1b17765' as 'base16-shell'