Make the zeromq filter actualy work

Two changes:
1) The existing code calls JSON.parse() without importing json. Fixed.
2) The existing code calls event.overwrite() with a Hash param, rather than a LogStash::Event.  The overwrite() method expects an Event, so this can't work (see code in event.rb).  The zeromq server can only reply with a JSON object, so the Hash reply from JSON.parse() needs to be made into an Event before being used in overwrite()

I'm new to both Logstash and Ruby, but the zeromq filter didn't work at all as is, and works with this patch.