build: switch to C++17

Change-Id: Id6217b5c993f3e4726e89773128b565e5f136bb6
diff --git a/tests/key-chain-fixture.cpp b/tests/key-chain-fixture.cpp
index 283c7bc..59098d7 100644
--- a/tests/key-chain-fixture.cpp
+++ b/tests/key-chain-fixture.cpp
@@ -23,8 +23,7 @@
 
 #include <boost/filesystem.hpp>
 
-namespace ndn {
-namespace tests {
+namespace ndn::tests {
 
 using namespace ndn::security;
 
@@ -113,5 +112,4 @@
   return saveIdentityCert(id, filename);
 }
 
-} // namespace tests
-} // namespace ndn
+} // namespace ndn::tests