Fix an issue with `--debug` and `_dump` on Ruby 1.8.7

This fixes an issue where both the `--debug` option and the `brew cask _dump` command would fail on Ruby 1.8.7 when the Cask has an `installer` stanza, e. g. `adobe-air`.
Merge pull request #7893 from claui/fix-brew-cask-dump-ruby18

Fix an issue with `--debug` and `_dump` on Ruby 1.8.7