fix build with boost 1.57
fix build with boost 1.57

this is the fix applied in homebrew and in

  https://github.com/luabind/luabind/issues/27

Conflicts:
	luabind/detail/object.hpp

See also rpavlik/luabind#23.

Oberon00/luabind,Oberon00/luabind,lonski/luabind