Change sdk detection to same syntax used elsewhere.

This wasn't working for me on leopard with ruby 1.9.2.

Closes Homebrew/homebrew#14636.

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

Library/Homebrew/macos.rb

@@ -75,7 +75,7 @@ module MacOS extend self
       opts << "#{Xcode.prefix}/Platforms/MacOSX.platform/Developer/SDKs/MacOSX#{v}.sdk"
       # Xcode < 4.3 style
       opts << "/Developer/SDKs/MacOS#{v}.sdk"
-      opts.map{|a| Pathname.new(a) }.detect(&:directory?)
+      opts.map{|a| Pathname.new(a) }.detect { |p| p.directory? }
     end
   end