Require Visual Studio 2015 Update 3 or later

fix #216

test/CMakeLists.txt

@@ -8,6 +8,7 @@ add_executable (lib-tests
     ${BEAST_INCLUDES}
     ${EXTRAS_INCLUDES}
     ../extras/beast/unit_test/main.cpp
+    config.cpp
     core.cpp
     http.cpp
     version.cpp

test/Jamfile

@@ -7,6 +7,7 @@
 
 import os ;
 
+compile config.cpp : : ;
 compile core.cpp : : ;
 compile http.cpp : : ;
 compile version.cpp : : ;

test/config.cpp

@@ -0,0 +1,9 @@
+//
+// Copyright (c) 2013-2017 Vinnie Falco (vinnie dot falco at gmail dot com)
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+//
+
+// Test that header file is self-contained.
+#include <beast/config.hpp>