Improve and simplify code with modern C++ features

Change-Id: I28d84df3087492ab2ecbeb91169a2cde12c9e31e
diff --git a/src/delegation-list.cpp b/src/delegation-list.cpp
index e21d100..91ea7af 100644
--- a/src/delegation-list.cpp
+++ b/src/delegation-list.cpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /*
- * Copyright (c) 2013-2017 Regents of the University of California.
+ * Copyright (c) 2013-2018 Regents of the University of California.
  *
  * This file is part of ndn-cxx library (NDN C++ library with eXperimental eXtensions).
  *
@@ -33,7 +33,7 @@
 }
 
 DelegationList::Error::Error(const std::string& what, const std::exception& innerException)
-  : Error(what + std::string(": ") + innerException.what())
+  : Error(what + ": "s + innerException.what())
 {
 }