time: Adapt to ndn-cpp-dev's new time structure

Change-Id: I4117d4e7aea5347cbaf3ec2e5e5a110f94930262
diff --git a/src/invitation.cpp b/src/invitation.cpp
index 4aa3358..851010b 100644
--- a/src/invitation.cpp
+++ b/src/invitation.cpp
@@ -59,7 +59,7 @@
   , m_chatroom(chatroom)
   , m_inviterRoutingPrefix(inviterRoutingPrefix)
   , m_inviterCertificate(inviterCertificate)
-  , m_timestamp(time::now())
+  , m_timestamp(time::toUnixTimestamp(time::system_clock::now()).count())
 {
   m_interestName = m_inviteeNameSpace;
   m_interestName.append("CHRONOCHAT-INVITATION")