Use double quotes.

Library/Homebrew/cask/spec/formatter_spec.rb

@@ -5,10 +5,10 @@ describe Formatter do
   describe "::columns" do
     let(:input) {
       [
-        'aa',
-        'bbb',
-        'ccc',
-        'dd'
+        "aa",
+        "bbb",
+        "ccc",
+        "dd",
       ]
     }
     subject { described_class.columns(input) }