Stick to rufus-scheduler 2.0.x versions.

Unbreaks tests as seen in
https://travis-ci.org/logstash/logstash/jobs/12047597.
Merge pull request #687 from bernd/rufus-scheduler-version

Stick to rufus-scheduler 2.0.x versions
- Add more tests to verify document index and index type settings
- copy tests from elasticsearch_http to test type settings
Merge git://github.com/logstash/logstash