Fix binary compatibility of 0.5 branch with node v0.10.x

The 0.5 branch of this node-ffi project is still up on npm with its own alias as `node-ffi`, which exposes the 0.x API of this project.  Several projects, notably NodObjC are still dependent on this older API.  I agree that they should ideally move to the newer 1.x API, but in the meantime, this should get everyone going.