Bump version 1.3.1

Change-Id: Id4c79892bdc0d0bc73b6d07496ed4b58f18d51cf
Reviewed-on: http://review.couchbase.org/26758
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Matt Ingenthron <matt@couchbase.com>