The Elasticsearch backend checks to make sure the elasticsearch module

is installed, but doesn't check its version. Haystack only supports the

1.x and 2.x series, and people these days are going to get much newer

versions than that. We now check the version of the module, and provide

an error message stating the version requirements of both the module and

the service.

Tested without the module, with a 6.x version of the module, and a 2.x

version. I saw the error in the first two conditions, and validation

passed in the last.
Unit tests pass.