Revert logstash sensu handler

Sensu community plugins updated the logstash handler to use a message
only accepted by logstash 1.2, not 1.1 which we use. This is the
previous working version, which is vendored here rather than
forking sensu-community-plugins again.

See
https://github.com/sensu/sensu-community-plugins/commit/b2257d7e8a17696fb7d0f1451a7ce2fe5c14320f#diff-e6d2ad1f8e19a38c7f6e8c1e87d7961d
and
https://logstash.jira.com/browse/LOGSTASH-675
Merge pull request #490 from alphagov/sensu-alerts-back-in-logstash

Revert logstash sensu handler