Fix Vacant/Occupied imports

I'm getting the following with a bleeding rustc (`rustc 0.13.0-dev 658529467`)

    src/scene.rs:92:43: 92:49 error: unresolved import `std::collections::hash_map::Vacant`. There is no `Vacant` in `std::collections::hash_map`
    src/scene.rs:92         use std::collections::hash_map::{ Vacant, Occupied };
                                                              ^~~~~~
    src/scene.rs:92:51: 92:59 error: unresolved import `std::collections::hash_map::Occupied`. There is no `Occupied` in `std::collections::hash_map`
    src/scene.rs:92         use std::collections::hash_map::{ Vacant, Occupied };
                                                                      ^~~~~~~~

I figured the author meant the import to be `std::collections::hash_map::Entry`. At least that works on my system.