sheep: save updated config after version migration

Current config migration mechanism doesn't save updated version number
even after successful migration. This patch fixes the problem.

Related issue:
https://bugs.launchpad.net/sheepdog-project/+bug/1327040

Cc: Ruoyu <liangry@ucweb.com>
Cc: Giovanni Bellac <giovannib1979@ymail.com>
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>

Conflicts:
	sheep/config.c

Conflicts were resolved by Hitoshi Mitake.
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>