Replace deprecated _all in SuggestionsAccessor

[Xrampino]

_all field is deprecated in ES 6.0.0. See https://www.elastic.co/guide/en/elasticsearch/reference/current/mapping-all-field.html

As such, direct_generator, referenced in SuggestionsAccessor, is no longer operational. This PR adresses this, by using directly the current field instead. Since there is no longer a standard catchall field, keeping the generation on a catchall field was not possible.

[CLAassistant]

All committers have signed the CLA.

[coveralls]

Coverage remained the same at 98.69% when pulling c3bc51b on Xrampino:develop into e0df076 on searchkit:develop.

[ssetem]

hi @Xrampino

Thanks for the PR, do you know if this is backwards compatible with es5 etc?

[Xrampino]

Sure, I forgot to tell it in my PR description, but this can be any field, back since ES2 : https://www.elastic.co/guide/en/elasticsearch/reference/2.0/search-suggesters-phrase.html

field : The field to fetch the candidate suggestions from.

[ssetem]

nice @Xrampino thanks

[Milananas]

Is there already a stable release with this change? Or is there a way around it in the current release?

[christopher-johnson]

@ssetem
can you cut a new release with this commit please? Thank you