Use sort_by in PrettyListing

Library/Homebrew/cmd/list.rb

@@ -85,7 +85,7 @@ end
 
 class PrettyListing
   def initialize path
-    Pathname.new(path).children.sort{ |a,b| a.to_s.downcase <=> b.to_s.downcase }.each do |pn|
+    Pathname.new(path).children.sort_by { |p| p.to_s.downcase }.each do |pn|
       case pn.basename.to_s
       when 'bin', 'sbin'
         pn.find { |pnn| puts pnn unless pnn.directory? }