build: use C++11

Change-Id: I0e58ac4e9cb42d07a9b58125d761875f91c7744c
Refs: #1930
diff --git a/src/handles/watch-handle.cpp b/src/handles/watch-handle.cpp
index 15a82fa..9e80212 100644
--- a/src/handles/watch-handle.cpp
+++ b/src/handles/watch-handle.cpp
@@ -98,7 +98,8 @@
 }
 
 void
-WatchHandle::onValidationFailed(const shared_ptr<const Interest>& interest, const string& reason)
+WatchHandle::onValidationFailed(const shared_ptr<const Interest>& interest,
+                                const std::string& reason)
 {
   std::cerr << reason << std::endl;
   negativeReply(*interest, 401);