remove String#undent 🐵-patch

[dduugg]

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If I'm understanding the use & intent correctly, undent can be replaced with the squiggly heredoc introduced in Ruby 2.3.

[apainintheneck]

Makes sense to me. It looks like this method was added when the rubocop-cask gem got merged into the main brew repo in #5190. I assume it was originally used instead of what you changed it to because this code was probably written pre-Ruby 2.3.

[apainintheneck]

Glad to get rid of a monkey patch. Thanks, @dduugg!

[MikeMcQuaid]

Thanks again @dduugg!