Enable elasticsearch-curator on elasticsearch 7
Enable elasticsearch-curator on elasticsearch 7

The current nixpkgs use elasticsearch-curator 5.8.1. As of version 5.7.0,
elasticsearch-curator supports elasticsearch 7, thus this change enables tests
with ES 7.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs