Updating check to make Boost.Asio think CE is in use to be only for before VS2015.

Author: stgates

Release/tests/functional/websockets/utilities/stdafx.h

@@ -32,11 +32,13 @@
 
 #if defined(_WIN32)
 // Trick Boost.Asio into thinking CE, otherwise _beginthreadex will be used which is banned
-// for the Windows Runtime. Then CreateThread will be used instead.
+// for the Windows Runtime pre VS2015. Then CreateThread will be used instead.
+#if _MSC_VER < 1900
 #if defined(__cplusplus_winrt)
 #define UNDER_CE 1
 #endif
 #endif
+#endif
 
 #include "cpprest/uri.h"
 #include "cpprest/asyncrt_utils.h"