l10n_fr: remove dependency on l10n_fr_rib

I think we should not oblige France-based users to install l10n_fr_rib any more. Since August 2014, all credit transfers and direct debits use SEPA, so everybody communicates bank account numbers with IBAN/BIC and not with RIB any more. So most users should be fine with support for IBAN only, and don't need support for RIB any more. Those who want to continue to have support for RIB could still install the module l10n_fr_rib manually.

My vision is:
- for v8: keep l10n_fr_rib in the official addons but make it optional (it's the goal of this PR)
- for v9: get l10n_fr_rib out of the official addons (and move it to community addons if some users want to continue to use it) (should be done in another PR)