fix timestamp test hard coding for UTC-7

When I ran the test, because I was in the eastern timezone, it failed with:

```
Failures:

  1) LogStash::Outputs::Kafka should send logstash event to kafka broker
     Failure/Error: kafka.receive(e)
       #<Kafka::Producer:0x6dc4a8dd> received :sendMsg with unexpected arguments
         expected: ("test", nil, "{\"message\":\"hello world\",\"host\":\"test\",\"@timestamp\":\"2014-07-01T17:08:12.544-07:00\",\"@version\":\"1\"}")
              got: ("test", nil, "{\"message\":\"hello world\",\"host\":\"test\",\"@timestamp\":\"2014-07-01T20:08:12.544-04:00\",\"@version\":\"1\"}")
     # ./lib/logstash/outputs/kafka.rb:65:in `register'
     # ./lib/logstash/codecs/json.rb:45:in `encode'
     # ./lib/logstash/outputs/kafka.rb:81:in `receive'
     # ./spec/outputs/kafka.rb:32:in `(root)'
     # ./lib/logstash/runner.rb:154:in `run'

Finished in 18.48 seconds
432 examples, 1 failure
```

This should fix it for 1.4.0.