cpp_redis: update to a version with the CMAKE_CXX_STANDARD fixed

This updates the cpp_redis submodule to a version
which contains https://github.com/ceph/cpp_redis/pull/5

Signed-off-by: Leonid Usov <leonid.usov@ibm.com>

diff --git a/src/cpp_redis b/src/cpp_redis
index c659475ea43bc77850018aa1433d55cad902ea85..72d992fff2a95edb37430a75909a844637549331 160000 (submodule)
--- a/src/cpp_redis
+++ b/src/cpp_redis
@@ -1 +1 @@
-Subproject commit c659475ea43bc77850018aa1433d55cad902ea85
+Subproject commit 72d992fff2a95edb37430a75909a844637549331