use ndn::optional instead of boost::optional

Change-Id: If4db620ac410ddf5d8a082b26bdc8fef0b4c1b46
diff --git a/src/requester.hpp b/src/requester.hpp
index b3c7901..cb3d10d 100644
--- a/src/requester.hpp
+++ b/src/requester.hpp
@@ -61,7 +61,7 @@
    * @return the CaProfile if decoding is successful
    * @throw std::runtime_error if the decoding fails or receiving an error packet.
    */
-  static boost::optional<CaProfile>
+  static optional<CaProfile>
   onCaProfileResponse(const Data& reply);
 
   /**
@@ -76,7 +76,7 @@
    * @return the CaProfile if decoding is successful
    * @throw std::runtime_error if the decoding fails or receiving an error packet.
    */
-  static boost::optional<CaProfile>
+  static optional<CaProfile>
   onCaProfileResponseAfterRedirection(const Data& reply, const Name& caCertFullName);
 
   /**