Update linux.rst

Elasticsearch install instructions. Changed paragraph about Ubuntu not providing a package and that the most recent, stable version should be used. This is contrary to what the IMPORTANT note below it states, about AtoM only working with 0.90.11. I fell foul of this by jumping ahead and downloading/installing the latest version off the Elasticsearch web site, only to read further on that the newer version isn't supported. My wording could perhaps be improved, but you get the idea.