fix to flush a buffer by USR1 signal even on error happening

Continued from https://github.com/fluent/fluentd/pull/468
-1 is because of condition

```ruby
              if @next_retry_time >= time
                # allow retrying for only one thread
                return time + @try_flush_interval
              end
```

https://github.com/fluent/fluentd/blob/4a523dd774d95c674280237fdcec8bf42e878cd1/lib/fluent/output.rb#L304

`@next_retry_time = Engine.now` will not make `@buffer.pop(self)` call immediately.