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)

It goes in a long chain of calls, from our code, to task, celery, etc. Given that we don't want to fail the whole reindex if there is some error, I would keep as is, and managed more detailed exceptions if we feel the need.

Tell me if it's ok for you.