use ndn::optional instead of boost::optional

Change-Id: If4db620ac410ddf5d8a082b26bdc8fef0b4c1b46
diff --git a/tools/ndncert-client.cpp b/tools/ndncert-client.cpp
index fc074a0..0807edc 100644
--- a/tools/ndncert-client.cpp
+++ b/tools/ndncert-client.cpp
@@ -190,7 +190,7 @@
 static void
 InfoCb(const Data& reply, const Name& certFullName)
 {
-  boost::optional<CaProfile> profile;
+  optional<CaProfile> profile;
   try {
     if (certFullName.empty()) {
       profile = Requester::onCaProfileResponse(reply);