fix encode (output) for oldlogstashjson codec.

It was referencing the obj variable which didn't exist
Write a failing test when prefix & include/exclude keys are used together
Fix a bug where field goes missing if include/exclude is used with prefix
Fixes LOGSTASH-1328
Fix fail() invocation if no config file is given.
Stick to rufus-scheduler 2.0.x versions.

Unbreaks tests as seen in
https://travis-ci.org/logstash/logstash/jobs/12047597.
Make ES index requests synchronous. Prior to this, I had incorrectly
assumed that elasticsearch acting as a client had a fixed-size thread
pool to limit the number of active bulk index requests. This now
executes bulk index requests one at a time.
Unbreak #to_epoch method in datadog output plugin. (LOGSTASH-1427)

The event timestamp might already be a Time object.
Use latest kibana for the flatjar. (LOGSTASH-1416)
Merge pull request #678 from jippi/patch-1

Fixes LOGSTASH-1328
Merge pull request #688 from logstash/synchronous-elasticsearch

Make ES index requests synchronous. Prior to this, I had incorrectly
rollback #678 it turns out this is really a bug in the ftw gem.
version bump ftw gem
Merge pull request #695 from logstash/es_http-fix

Fix for elasticsearch_http output
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
host is required
- Avoid generating piles of temporary strings simply to add a newline to
  things.
- force body to a string (was an array, join with "") to make FTW send a
fixed-size request (avoid chunking).

The idea here is to avoid the previous behavior that logstash sends 2
chunks per event to index (once for bulk header, once for document).
Many small chunks isn't very efficient, and until we make a more
efficient chunk sender, this will suffice.