Review Board no longer depends on the elasticsearch package. Instead,

we now provide ReviewBoard[elasticsearchX] packages for each supported

major version of Elasticsearch.
The instructions on the settings page reference these dependencies,

easing installation a bit, especially if we need to bump or cap versions

down the road.
Documentation has been updated to list the installation requirements,

and to list these packages.

Tested installing the dependency wrapper packages.
Verified the text on the configuration page and in the documentation.