Cargo/Travis warnings, tests, and device `CommandBuffer`

I thought of it to be used in a typical scenario:
```rust
fn init(device: &mut gfx::ActualDevice) {..}
```
But then since I removed the type parameter for the `Device` trait, we can write it like that instead:
```rust
fn init<D: gfx::Device>(device: &mut D) {
```
So I guess we can remove the `ActualDevice` thing.