Fix log rotation (send the correct signal post rotation)

I just noticed that agent logs on my server are empty.

It happens that we are sending a wrong signal to the agent after the log rotation (`SIGUSR1` instead of `SIGHUP`) which means agent doesn't re-open a new log file after the rotation and keeps writing to the old file.

Reference: https://github.com/racker/virgo-base/blob/master/lib/lua/virgo_init.lua#L465