Updated to version 2.4 and added c++11 option

I modified the formula so that the url now points to version 2.4
released November 2, 2013. In addition, I added the option to build
using C++11 mode.

Closes Homebrew/homebrew#24216.

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

Formula/nlopt.rb

@@ -2,10 +2,13 @@ require 'formula'
 
 class Nlopt < Formula
   homepage 'http://ab-initio.mit.edu/wiki/index.php/NLopt'
-  url 'http://ab-initio.mit.edu/nlopt/nlopt-2.3.tar.gz'
-  sha1 '28253b65187d9d1d4c75e96310d8ee8c9c5f3cfc'
+  url 'http://ab-initio.mit.edu/nlopt/nlopt-2.4.tar.gz'
+  sha1 'e766f4c49fa5923fb45220f278c01c04c38fc369'
+
+  option :cxx11
 
   def install
+    ENV.cxx11 if build.cxx11?
     system "./configure", "--prefix=#{prefix}"
     system "make"
     system "make install"