README: We require 3.5 master at the moment.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>
riemann: Upgrade to latest 3.5 master

log_threaded_dest_driver_init_method() was renamed to
log_threaded_dest_driver_start(), follow up the change here too.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>