Update goprotobuf repo path

Change goprotobuf repo path to a couchbaselabs mirror

Change-Id: If29d186816609862de400dd3cbaf7ae3918e4f70
Reviewed-on: http://review.couchbase.org/41580
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Tested-by: Sarath Lakshman <sarath@couchbase.com>