ringaccount: use Gzip to store unencrypted account archive

Using Gzip allows unencrypted archives to be opened and manipulated by users/scripts
using standard Gzip tools.

Change-Id: I913351b8bf97a6d69b531386511fb03ce86ce67d
Reviewed-by: Guillaume Roguez <guillaume.roguez@savoirfairelinux.com>