Fix install <cask> --dry-run

[danielbayley]

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---

Fixes the following error, when running brew install --dry-run:

> brew install <cask> --dry-run
==> Would install 1 cask:
<cask>
Error: uninitialized constant Cask::Cmd::Install::CaskDependent
/usr/local/Homebrew/Library/Homebrew/cask/cmd/install.rb:94:in `block in install_casks'
/usr/local/Homebrew/Library/Homebrew/cask/cmd/install.rb:93:in `each'
/usr/local/Homebrew/Library/Homebrew/cask/cmd/install.rb:93:in `install_casks'
/usr/local/Homebrew/Library/Homebrew/cmd/install.rb:192:in `install'
/usr/local/Homebrew/Library/Homebrew/brew.rb:95:in `<main>'

[apainintheneck]

LGTM

[MikeMcQuaid]

Good catch thanks again @danielbayley!