[elasticsearch_river][Fix] use mq.publish_serialized instead of receive_raw

receive_raw is no longer available in RabbitMQ.

elasticsearch_river throw the exception "undefined method `receive_raw'"

This commit fix issues 1298 and 1384
Fixed varname for endpoint_region
Merge pull request #712 from jujugrrr/s3-output-varname

Fixed varname for endpoint_region
Merge pull request #672 from vspiewak/patch-1

[elasticsearch_river] Fix undefined method receive_raw
Merge git://github.com/logstash/logstash