Update go.mod to latest gocbcore
Update go.mod to latest gocbcore

Change-Id: I278321394d3069ae1dd381fecbce8b976466ee8c
Reviewed-on: http://review.couchbase.org/115062
Reviewed-by: Ellis Breen <9d8041ac7ead8d3e1f3217c04c7ed5e3098c2381@couchbase.com>
Tested-by: Charles Dixon <e13bf4498b4994e133fd3586350658dc4a59329a@gmail.com>

brett19/gocb