fix test_object failing on lua 5.1

Fix this by using a proper implementation of  luaL_tolstring() for Lua
5.1. This code is from https://github.com/hishamhm/lua-compat-5.2
fix warning "old-style cast"

Clang 3.3 emits the following warning:

.../luabind/lua_include.hpp:59:37: warning: use of old-style cast [-Wold-style-cast]
     lua_pushlightuserdata(L, (void*)p);
                                     ^