use ndn::optional instead of boost::optional

Change-Id: If4db620ac410ddf5d8a082b26bdc8fef0b4c1b46
diff --git a/src/requester.cpp b/src/requester.cpp
index 8ae64eb..af3f6c4 100644
--- a/src/requester.cpp
+++ b/src/requester.cpp
@@ -62,7 +62,7 @@
   return interest;
 }
 
-boost::optional<CaProfile>
+optional<CaProfile>
 Requester::onCaProfileResponse(const Data& reply)
 {
   auto caItem = InfoEncoder::decodeDataContent(reply.getContent());
@@ -74,7 +74,7 @@
 }
 
 
-boost::optional<CaProfile>
+optional<CaProfile>
 Requester::onCaProfileResponseAfterRedirection(const Data& reply, const Name& caCertFullName)
 {
   auto caItem = InfoEncoder::decodeDataContent(reply.getContent());