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)

Before the olympia/zamboni split I was thinking this file should no longer exist and instead we should use elasticutils. The reason being that this file is actually what spawned elasticutils to exist and it has gone through many improvements since being created. And the more people sharing a lib the better it is which is good for all.

However, now elasticutils is at a crossroads where it isn't compatible with Elasticsearch 1.0+ and doesn't have some of these features (like custom score queries you've added). And the elasticsearch DSL will be released possibly replacing the need for elasticutils.

I need to make this same change in zamboni (add the custom scoring) so we can upgrade to 1.0+ but we're in a slightly different boat since we stopped using this file long ago.

There's nothing actionable in this comment -- I'm just brain dumping. If your plan is to use the elasticsearch DSL in the future when it is ready I support this change. :+1: