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)

> I think it'd be best to keep these versions matching as close as possible.

You mean like now, with MKT on 1.2 and AMO on 0.90? :p

Honestly, I don't think we really want that, because our work/push schedules are async, and thus I don't think AMO should wait for MKT to upgrade to 1.3 and neither MKT should wait for AMO to do so ;)

Let's try all of us to be up to date as much as possible and we will be as closer as possible :)