Automatically install the latest release of ElixirLS

[mhanberg]

It currently is hard coded to install v0.9.0, but it should just install the latest stable release.

[mhanberg]

This can be accomplished in the same way that it is done for Elixir.

It think that we can also switch to using Mix.install as well. This would probably remove some features, like being able to set the repo, branch and tag to use to compile. we can set the version instead, and if you want to run a local version, you can do that, but it will have to be manual.

this is a reasonable tradeoff for the maintenance burden