Fix to use time.to_i for checking when chunks should be flushed

[tagomoris]

I commented in code comment why this change is needed.

[tagomoris]

This change fixes the problem commented in #928 .
before:

2016-05-18 11:50:11 +0900 fluentd.traffic: {"test_count":300,"test_count_rate":300.0}
2016-05-18 11:50:12 +0900 fluentd.traffic: {"test_count":0,"test_count_rate":0.0}
2016-05-18 11:50:13 +0900 fluentd.traffic: {"test_count":600,"test_count_rate":600.0}
2016-05-18 11:50:14 +0900 fluentd.traffic: {"test_count":0,"test_count_rate":0.0}
2016-05-18 11:50:15 +0900 fluentd.traffic: {"test_count":570,"test_count_rate":570.0}
2016-05-18 11:50:16 +0900 fluentd.traffic: {"test_count":0,"test_count_rate":0.0}
2016-05-18 11:50:17 +0900 fluentd.traffic: {"test_count":600,"test_count_rate":600.0}
2016-05-18 11:50:18 +0900 fluentd.traffic: {"test_count":0,"test_count_rate":0.0}
2016-05-18 11:50:19 +0900 fluentd.traffic: {"test_count":570,"test_count_rate":570.0}
2016-05-18 11:50:20 +0900 fluentd.traffic: {"test_count":0,"test_count_rate":0.0}
2016-05-18 11:50:21 +0900 fluentd.traffic: {"test_count":570,"test_count_rate":570.0}
2016-05-18 11:50:22 +0900 fluentd.traffic: {"test_count":0,"test_count_rate":0.0}
2016-05-18 11:50:23 +0900 fluentd.traffic: {"test_count":720,"test_count_rate":720.0}

After:

2016-05-18 12:01:14 +0900 fluentd.traffic: {"test_count":300,"test_count_rate":300.0}
2016-05-18 12:01:15 +0900 fluentd.traffic: {"test_count":300,"test_count_rate":300.0}
2016-05-18 12:01:16 +0900 fluentd.traffic: {"test_count":300,"test_count_rate":300.0}
2016-05-18 12:01:17 +0900 fluentd.traffic: {"test_count":270,"test_count_rate":270.0}
2016-05-18 12:01:18 +0900 fluentd.traffic: {"test_count":300,"test_count_rate":300.0}
2016-05-18 12:01:19 +0900 fluentd.traffic: {"test_count":300,"test_count_rate":300.0}
2016-05-18 12:01:20 +0900 fluentd.traffic: {"test_count":210,"test_count_rate":210.0}
2016-05-18 12:01:21 +0900 fluentd.traffic: {"test_count":90,"test_count_rate":90.0}