poke and pingserver: use new signing API and helpers

refs #2919

Change-Id: I0770fc8ee553448f30b2e7a3ba902add441b9659
diff --git a/tools/ping/server/ping-server.cpp b/tools/ping/server/ping-server.cpp
index 699da5b..f919113 100644
--- a/tools/ping/server/ping-server.cpp
+++ b/tools/ping/server/ping-server.cpp
@@ -66,7 +66,7 @@
   shared_ptr<Data> data = make_shared<Data>(interestName);
   data->setFreshnessPeriod(m_options.freshnessPeriod);
   data->setContent(m_payload);
-  m_keyChain.signWithSha256(*data);
+  m_keyChain.sign(*data, signingWithSha256());
   m_face.put(*data);
 
   ++m_nPings;