lua converters: revert to boost::cref

Using std::cref causes a Lua error "Trying to use unregistered class".
The reason is that luaponte (luabind) is C++98 and binds only
boost::cref, see libs/luaponte/luaponte/detail/convert_to_lua.hpp.

The bug was introduced in commit:4b16bd3, which was not properly tested.
This fixes #335919.