Link to compose from old version of boost.

Compose isn't included in modular boost, so there's nothing to link to.
I changed it to link to an old version which still has compose
documentation. It might be better to rewrite it?

mem_fn.html

@@ -145,7 +145,7 @@ void k(std::vector<boost::shared_ptr<X> > const & v)
 		<p>
 			Yes. For simple uses, <b>mem_fn</b> provides additional functionality that the 
 			standard adaptors do not. Complicated expressions that use <b>std::bind1st</b>, <b>std::bind2nd</b>
-			or <a href="../compose/index.htm"><b>Boost.Compose</b></a> along with the 
+			or <a href="http://www.boost.org/doc/libs/1_31_0/libs/compose/index.htm"><b>Boost.Compose</b></a> along with the 
 			standard adaptors can be rewritten using <a href="bind.html"><b>boost::bind</b></a>
 			that automatically takes advantage of <b>mem_fn</b>.
 		</p>