Avoid deprecated Boost.Asio interfaces

Change-Id: I7041c89ea9147e08c8b6226b84a6d17dddeed0e1
diff --git a/tests/unit/read-handle.t.cpp b/tests/unit/read-handle.t.cpp
index 9d36b18..553f651 100644
--- a/tests/unit/read-handle.t.cpp
+++ b/tests/unit/read-handle.t.cpp
@@ -44,7 +44,7 @@
 public:
   Fixture()
     : face({true, true})
-    , scheduler(face.getIoService())
+    , scheduler(face.getIoContext())
     , subsetLength(1)
     , dataPrefix("/ndn/test/prefix")
     , identity("/ndn/test/identity")