Upgrading Elasticsearch (bug 1013874)

Ref https://bugzilla.mozilla.org/show_bug.cgi?id=1013874

This also move from pyes/pyelasticsearch/elasticutils to official elasticsearch lib.
Next step is to move out from our custom DSL to the official one, when it lands.

Main changes:
- `_boost` is no more a special field, we need to use a `function_score` query instead
- `_source.xxx` in `fields` is deprecated
- `fields` parameter doesn't allow anymore non leaf fields (so `_sources.versions` will not work, as it's a list of objects)

> Just like Mozilla, soccer promotes healthy competition, encourages fair play and defends
> standards to level the playing field.

That's definitively the plan! :)
I've seen that you already jumped into elasticsearch-dsl, but for the record:
> ybon | honzakral: hi :) Any rough idea about some release of elasticsearch-dsl-py?
> honzakral | ybon: the query builder is mostly ok, I am waiting for some feedback and trying to write docs for it before I release