Lua 5.2 compatibility

Simply changed `luaL_reg` to `luaL_Reg` everywhere. Fixes #4. 

(Note: still not compatible with Lua 5.2 without LUA_COMPAT_MODULE flag(which is a must for getting luarocks <= 2.1.2 working anyway))

(Note: as [with the scm rockspec](https://github.com/agladysh/lua-hiredis/commit/e59e255ec8ec2a63b504c58f05b3ec8b9a2bdbc1), `lib/` should be added to include directories in the new rockspec, if a release happens)