generators: update active_merchant templates

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
payment_plugin: fix save_response_and_transaction call

Make sure to pass the right response model.

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
VERSION: bump to 3.1.6

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>