Bump version to v0.1.8 (#566)

This also bumps a number of sub crates:

* tokio-executor (0.1.3)
* tokio-io (0.1.8)
* tokio-reactor (0.1.4)
* tokio-threadpool (0.1.6)
* tokio-timer (0.2.6)
* tokio-udp (0.1.2)

tokio-io/src/lib.rs

@@ -1,3 +1,6 @@
+#![deny(missing_docs, missing_debug_implementations, warnings)]
+#![doc(html_root_url = "https://docs.rs/tokio-io/0.1.8")]
+
 //! Core I/O traits and combinators when working with Tokio.
 //!
 //! A description of the high-level I/O combinators can be [found online] in
@@ -6,9 +9,6 @@
 //! [found online]: https://tokio.rs/docs/getting-started/core/
 //! [low level details]: https://tokio.rs/docs/going-deeper-tokio/core-low-level/
 
-#![deny(missing_docs, missing_debug_implementations, warnings)]
-#![doc(html_root_url = "https://docs.rs/tokio-io/0.1.7")]
-
 #[macro_use]
 extern crate log;