build: recommend boost >= 1.65.1

With this commit, Ubuntu 16.04 is no longer officially supported
and may require manual installation of a newer version of Boost.

Refs: #5087
Change-Id: I17197f72ee8f7cd4d0954658fe57c324076f1311
diff --git a/tests/unit/util/placeholders2.t.cpp b/tests/unit/util/placeholders2.t.cpp
index d8b8c93..7b71406 100644
--- a/tests/unit/util/placeholders2.t.cpp
+++ b/tests/unit/util/placeholders2.t.cpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /*
- * Copyright (c) 2013-2018 Regents of the University of California.
+ * Copyright (c) 2013-2020 Regents of the University of California.
  *
  * This file is part of ndn-cxx library (NDN C++ library with eXperimental eXtensions).
  *
@@ -21,13 +21,14 @@
 
 // Bug 2109 test case
 
-// interest.hpp includes common.hpp; common.hpp shouldn't be used from external program
+// interest.hpp includes common.hpp; common.hpp is an implementation detail and should not
+// be directly included from an external program
 #include "ndn-cxx/interest.hpp"
 
-#include <boost/bind.hpp>
+#include <boost/bind/bind.hpp>
 
 void
-placeholders2TestFunction(int i)
+placeholders2TestFunction(int)
 {
 }