LP#1162907: install RPC::XML as Evergreen dependency

RPC::XML used to be installed as part of installing
the OpenSRF deps, but since OpenSRF itself doesn't
use it, this belongs in Evergreen.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Signed-off-by: Mike Rylander <mrylander@gmail.com>