build: require gcc >= 5.3 and switch to C++14

Change-Id: I93c8fc238cc2654346bbc69a9d8ea83d1c0b099e
Refs: #3076, #4462
diff --git a/src/update/prefix-update-processor.cpp b/src/update/prefix-update-processor.cpp
index 26adefd..57bb579 100644
--- a/src/update/prefix-update-processor.cpp
+++ b/src/update/prefix-update-processor.cpp
@@ -54,8 +54,7 @@
                                              NamePrefixList& namePrefixList,
                                              Lsdb& lsdb)
   : CommandManagerBase(dispatcher, namePrefixList, lsdb, "prefix-update")
-
-  , m_validator(ndn::make_unique<ndn::security::v2::CertificateFetcherDirectFetch>(face))
+  , m_validator(std::make_unique<ndn::security::v2::CertificateFetcherDirectFetch>(face))
 {
   NLSR_LOG_DEBUG("Setting dispatcher to capture Interests for: "
     << ndn::Name(Nlsr::LOCALHOST_PREFIX).append("prefix-update"));