Disable Style/DoubleNegation cop

As discussed in bbatsov/rubocop#3344, there are situations where
this is the shortest and most idiomatic way to convert a value
to a boolean - particularly for checking values which can be
nil or truthy.

Library/Homebrew/.rubocop.yml

@@ -33,6 +33,10 @@ Style/BlockDelimiters:
   Exclude:
     - '**/*_spec.rb'
 
+# used idiomatically to return boolean values
+Style/DoubleNegation:
+  Enabled: false
+
 # so many of these in formulae but none in here
 Style/GuardClause:
   Enabled: true