Refactor brew cask style not to require .rubocop.yml.

[reitermarkus]

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Since rubocop-cask already includes a default.yml, we don't need to have a separate YAML file inside caskroom/cask. This also fixes the rare case that brew cask style fails when caskroom/cask is untapped, this is useful so we can have tests for brew cask style without having caskroom/cask tapped.

@Homebrew/cask

[reitermarkus]

Had to add --config /dev/null to make sure that only rubocop-cask's default.yml is used. Not sure if there is a better/official way to force using the default configuration.

[jawshooah]

@reitermarkus I went ahead and asked for you: rubocop/rubocop#3647

[jawshooah]

Merging for now since this works as-is, but will follow up in a separate PR if necessary once we hear back on rubocop/rubocop#3647.

[jawshooah]

@reitermarkus rubocop/rubocop#3658 has been merged, so once the next release is out we can replace --config /dev/null with --force-default-config.