Properly deinit() servo

[paulrouget]

servo::deinit needs to be called.

We get a crash on quit:

thread 'Constellation' panicked at 'Unexpected compositor channel panic in constellation: RecvError', src/libcore/result.rs:906:4
note: Run with `RUST_BACKTRACE=1` for a backtrace.
thread 'ResourceManager' panicked at 'IpcSelectionResult::unwrap(): channel 32259 closed', /Users/paul/.cargo/registry/src/github.com-1ecc6299db9ec823/ipc-channel-0.8.0/src/ipc.rs:308:16
thread 'LayoutThread PipelineId { namespace_id: PipelineNamespaceId(0), index: PipelineIndex(0) }' panicked at 'called `Result::unwrap()` on an `Err` value: RecvError', src/libcore/result.rs:906:4