Use Formula#installed_prefix for brew --prefix formula

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

Library/Homebrew/cmd/--prefix.rb

@@ -3,7 +3,7 @@ module Homebrew extend self
     if ARGV.named.empty?
       puts HOMEBREW_PREFIX
     else
-      puts ARGV.formulae.map{ |f| f.prefix }
+      puts ARGV.formulae.map{ |f| f.installed_prefix }
     end
   end
 end