src: Remove unnecessary uses of boost::cref in make_shared and replace boost::cref/boost::ref with just cref/ref

In some cases, due to argument-dependent lookup, it is necessary to use
ndn::ref, instead of just ref.

Change-Id: I682180a007609535855f77511b49622154ad4f11
Refs: #1591
diff --git a/core/scheduler.cpp b/core/scheduler.cpp
index fb09557..5f9e707 100644
--- a/core/scheduler.cpp
+++ b/core/scheduler.cpp
@@ -34,7 +34,7 @@
 getGlobalScheduler()
 {
   if (!static_cast<bool>(g_scheduler)) {
-    g_scheduler = make_shared<Scheduler>(boost::ref(getGlobalIoService()));
+    g_scheduler = make_shared<Scheduler>(ref(getGlobalIoService()));
   }
   return *g_scheduler;
 }