fix C++14 usage that brokes some builds

Some builds, like OSX and Debian 8, fails to build the daemon
as a patch has recently introduced a C++14 feature, but we don't
support C++14 by default on all platforms.

This patch changes the code to require only a C++11 compiler.

Change-Id: Ib858a6abddb610b6f94cedc01945c0de92f00687