fix travis

See https://travis-ci.org/fluent/fluentd/builds/10059116, travis is failing. 
This fixes the error although https://github.com/fluent/fluentd/pull/169 is still needed to pass all tests. 

FYI: This bug was embeded by this commit https://github.com/fluent/fluentd/commit/f463f8a1ef2448636f5cdb98261cbd6b2a024d4c