Disable elixir-sense's logging in production (#748)

Right now, this change is a no-op, but if and when this change
elixir-lsp/elixir_sense#165
lands, this will quiet elixir_sense's logging in the prod
environment, which will prevent logspam in lsp buffers when
elixir_sense encounters invalid code.

apps/language_server/lib/language_server/cli.ex

@@ -22,6 +22,7 @@ defmodule ElixirLS.LanguageServer.CLI do
 
     Launch.start_mix()
 
+    Application.put_env(:elixir_sense, :logging_enabled, Mix.env() != :prod)
     Build.set_compiler_options()
 
     start_language_server()