unpack moved to table.unpack in lua5.2

--- lua/cjson/util.lua.orig	Mon May 16 22:58:17 2016
+++ lua/cjson/util.lua	Mon May 16 23:05:46 2016
@@ -4,6 +4,8 @@ local json = require "cjson"
 --
 -- Mark Pulford <mark@kyne.com.au>
 
+local unpack = unpack or table.unpack
+
 -- Determine with a Lua table can be treated as an array.
 -- Explicitly returns "not an array" for very sparse arrays.
 -- Returns: