iproute: use the table 254 by default in get_default_routes()

Before this patch, get_default_routes() uses a default table value
of None, which causes an exception in encode() (netlink/generic.py
line 410) when trying to pack a None value.
Merge pull request #41 from rgacogne/master

iproute: use the table 254 by default in get_default_routes()