Add valgrind-3.6.0 support to unit tests

## Introduction
I got a machine with `valgrind-3.6.0.SVN-Debian`. Unfortunately, `Valgrind-3.6.0` doesn't have the `show-possibly-lost` flag causing the unit-test exits with the following error message:

    ERROR:valgrind: Bad option: --show-possibly-lost=no
    valgrind: Use --help for more information or consult the user manual.

## Notes
* I've assumed that any version older than 3.6.0 doesn't support the `show-possibly-lost` flag.
* I've added a simple `attention` message in case the machine has old Valgrind version.
* Feel free to share your thoughts.