Fix RuboCop offenses.

Library/.rubocop.yml

Library/Homebrew/cask/cask.rb

Library/Homebrew/cask/cmd.rb

Library/Homebrew/cask/cmd/abstract_command.rb

Library/Homebrew/cask/cmd/install.rb

Library/Homebrew/cask/dsl.rb

Library/Homebrew/cli/parser.rb

Library/Homebrew/compilers.rb

Library/Homebrew/dependency.rb

Library/Homebrew/extend/os/linux/formula.rb

Library/Homebrew/extend/os/mac/formula.rb

Library/Homebrew/formula.rb

Library/Homebrew/language/python.rb

Library/Homebrew/linkage_checker.rb

Library/Homebrew/requirement.rb

Library/Homebrew/resource.rb

Library/Homebrew/software_spec.rb

Library/Homebrew/tap.rb

Library/Homebrew/test/cmd/--cache_spec.rb

Library/Homebrew/test/cmd/--cellar_spec.rb