cleanup: fix for formula/cask name collision

[chasestubblefield]

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When a formula and cask share the same name, a download can be incorrectly considered stale. This is the case for the name docker:

$ brew cleanup
...
$ brew cask fetch docker
==> Downloading external files for Cask docker
==> Downloading https://download.docker.com/mac/stable/31259/Docker.dmg
######################################################################## 100.0%
==> Verifying SHA-256 checksum for Cask 'docker'.
==> Success! Downloaded to -> /Users/chase/Library/Caches/Homebrew/downloads/be9b9366a4a96478a04d2711baad17a8d5dc1342a82facd388f211b741965149--Docker.dmg
$ brew cleanup -n
Would remove: /Users/chase/Library/Caches/Homebrew/Cask/docker--2.0.0.3-ce-mac81,31259.dmg (521.5MB)
==> This operation would free approximately 521.5MB of disk space.

This is because the download for the cask docker (/Users/chase/Library/Caches/Homebrew/Cask/docker--2.0.0.3-ce-mac81,31259.dmg) is considered a stale download for the formula docker.

The fix is to call either stale_formula? or stale_cask? on the download based on the containing directory, but not both. Also, a check on the containing directory in stale_cask? is no longer necessary.

[MikeMcQuaid]

Thanks so much for your great first contribution (I hope to see more from you)! Without people like you submitting PRs we couldn't run this project. You rock, @Chasetopher !