Make unlock_gil true by default in C ctor

Change-Id: I8496117063f5460043bdd78eaaf8fd19fedec2b1
Reviewed-on: http://review.couchbase.org/26550
Tested-by: Mordechai Nunberg <mnunberg@haskalah.org>
Reviewed-by: Volker Mische <volker.mische@gmail.com>