Yet again an attempt to appease travis

src/gui/dialogs/end_credits.cpp

@@ -99,7 +99,7 @@ void tend_credits::pre_show(twindow& window)
 	// TODO: this seems an inefficient way to place the focused group first
 	if(!focus_str.str().empty()) {
 		focus_str << str.rdbuf();
-		str.swap(focus_str);
+		str = std::move(focus_str);
 	}
 
 	// Get the appropriate background images