Use CATCH_NULL instead of NULL

- expands to nullptr if CATCH_CONFIG_CPP11_NULLPTR is defined (see #444)

include/internal/catch_timer.hpp

@@ -37,7 +37,7 @@ namespace Catch {
 #else
         uint64_t getCurrentTicks() {
             timeval t;
-            gettimeofday(&t,NULL);
+            gettimeofday(&t,CATCH_NULL);
             return static_cast<uint64_t>( t.tv_sec ) * 1000000ull + static_cast<uint64_t>( t.tv_usec );
         }
 #endif