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)

Actually looking at the code there is something I do not understand: `model.index` and `search.get_es().flush_bulk` were using two distinct instances of `Elasticsearch`, and I don't see how the `es.bulker.bulk_data` was kept persistent between those two calls. Any hint?