Use boost::lexical_cast instead of std::to_string

fix #176

examples/http_example.cpp

@@ -7,6 +7,7 @@
 
 #include <beast/http.hpp>
 #include <boost/asio.hpp>
+#include <boost/lexical_cast.hpp>
 #include <iostream>
 #include <string>
 
@@ -25,7 +26,8 @@ int main()
     req.method = "GET";
     req.url = "/";
     req.version = 11;
-    req.headers.replace("Host", host + ":" + std::to_string(sock.remote_endpoint().port()));
+    req.headers.replace("Host", host + ":" +
+        boost::lexical_cast<std::string>(sock.remote_endpoint().port()));
     req.headers.replace("User-Agent", "Beast");
     beast::http::prepare(req);
     beast::http::write(sock, req);