Fix System.halt/1 and System.halt/0 type specifications

Dialyzer no longer complains about them.

Author: yrashk

lib/elixir/lib/system.ex

@@ -169,6 +169,8 @@ defmodule System do
       System.halt(:abort)
 
   """
+  @spec halt() :: no_return
+  @spec halt(non_neg_integer | List.Chars.t | :abort) :: no_return
   @spec halt(non_neg_integer | List.Chars.t | :abort, [] | [flush: false]) :: no_return
   def halt(status // 0, options // [])