mockserver: fix corrupt jar on py3

Change-Id: I1cc80a62bbcb16e298c31cb5167563d4ae2b5206
Reviewed-on: http://review.couchbase.org/29817
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>