Fix pretty names

Library/Homebrew/os/mac/version.rb

@@ -15,7 +15,15 @@ module MacOS
     end
 
     def pretty_name
-      @version.split('_').map(&:capitalize).join(' ')
+      case @version
+      when "10.9" then "Mavericks"
+      when "10.8" then "Mountain Lion"
+      when "10.7" then "Lion"
+      when "10.6" then "Snow Leopard"
+      when "10.5" then "Leopard"
+      when "10.4" then "Tiger"
+      else @version
+      end
     end
   end
 end