uses.rb: Pass a block to map instead of a Proc

Passing a `Proc` doesn't work in Ruby 1.8.6 which causes `brew uses` to fail on
Leopard.

Library/Homebrew/cmd/uses.rb

@@ -20,6 +20,6 @@ module Homebrew extend self
         keg.directory? and not keg.subdirs.empty?
       end
     end
-    puts_columns uses.map(&:to_s).sort
+    puts_columns uses.map{|f| f.to_s}.sort
   end
 end