Send SIGHUP to logstash after log rotation.

Logstash now reopens its log file on SIGHUP, starting in version [1.2.0](https://github.com/logstash/logstash/blob/46e5b75c3f821d3a2f4834312b1e75ab3ebcf3cb/CHANGELOG#L35).

I have confirmed, and it's safe to send a SIGHUP to older versions of LogStash as well (tested on 1.1.13).