CBLT-298:  set default iOS build-to-release

Change-Id: I708279d0a9686a66845dfac9cc1a174e30a54f8c
Reviewed-on: http://review.couchbase.org/37534
Reviewed-by: Philip Labee <philip@couchbase.com>
Tested-by: Philip Labee <philip@couchbase.com>