make elasticsearch example run out of the box

[miku]

Description

Change parameters to luigi.run in elasticsearch indexing example.

Motivation and Context

The --task parameter seems obsolete now.

error: unrecognized arguments: --task

Also, I tested the example with the latest elasticsearch release 5.0.0 - it works.

Have you tested this? If so, how?

Tested locally:

$ brew install elasticsearch
$ elasticsearch --daemonize
$ python examples/elasticsearch_index.py
...
This progress looks :)

[mention-bot]

@miku, thanks for your PR! By analyzing the history of the files in this pull request, we identified @Tarrasch, @steenzout and @themalkolm to be potential reviewers.

[Tarrasch]

An improvement so worth merging. Thanks @miku!

To improve it further, one could replace the whole code block with something like this