Homebrew has deprecated `brew cask cleanup` and `brew cask search` in favor
of `brew cleanup` and `brew search` respectively. They will stop working on
2018-09-30. Further, `--outdated` has been removed.
We should eventually remove the related aliases, but for a while we keep
supporting them gracefully with deprecation warning.
The documentation has been removed from README.md, however.
Indrajit Raychaudhuri