Bump Tokio to v0.1.15. (#869)

Also bumps:

- tokio-sync (0.1.0)
- tokio-threadpool (0.1.11)
- tokio-timer (0.2.9)

tokio-sync/README.md

@@ -0,0 +1,19 @@
+# tokio-sync
+
+Synchronization utilities
+
+[Documentation](https://docs.rs/tokio-sync/0.1.0/tokio_sync/)
+
+## Overview
+
+This crate provides synchronization utilities for usage with Tokio.
+
+## License
+
+This project is licensed under the [MIT license](LICENSE).
+
+### Contribution
+
+Unless you explicitly state otherwise, any contribution intentionally submitted
+for inclusion in Tokio by you, shall be licensed as MIT, without any additional
+terms or conditions.