Include config directory in gem file

Commit 4b13f23 forgot to add the new `config` directory to the gemspec,
resulting in default configurations not being found (and the gem
ultimately crashing).

Change-Id: Ieff25d0e002c9dae069c16e88685a9b802254d4d
Reviewed-on: https://gerrit.causes.com/31531
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>