Explicitly disable _all field
Explicitly disable _all field

It's not clear from the documentation if this is necessary for ES6,
but it can't hurt.  It does complain that the field is deprecated:

> [_all] is deprecated in 6.0+ and will be removed in 7.0. As a
> replacement, you can use [copy_to] on mapping fields to create your
> own catch all field.

So we'll probably just want to revert this commit when we switch to 7.

alphagov/rummager,alphagov/rummager