linkapps: replace #max_by with #max

Enumerable#max_by didn't exist in 1.8.6, so linkapps failed on Leopard.

Library/Contributions/cmd/brew-linkapps.rb

@@ -13,7 +13,7 @@ HOMEBREW_CELLAR.subdirs.each do |rack|
   kegs = rack.subdirs.map { |d| Keg.new(d) }
   next if kegs.empty?
 
-  keg = kegs.detect(&:linked?) || kegs.max_by(&:version)
+  keg = kegs.detect(&:linked?) || kegs.max {|a,b| a.version <=> b.version}
 
   Dir["#{keg}/*.app", "#{keg}/bin/*.app", "#{keg}/libexec/*.app"].each do |app|
     puts "Linking #{app}"