fixed elixir on-attach (#979)

lua/lang/elixir.lua

@@ -39,6 +39,7 @@ M.lsp = function()
 
   require("lspconfig").elixirls.setup {
     cmd = { DATA_PATH .. "/lspinstall/elixir/elixir-ls/language_server.sh" },
+    on_attach = require("lsp").common_on_attach,
   }
 end