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-executor/src/lib.rs

@@ -1,3 +1,6 @@
+#![deny(missing_docs, missing_debug_implementations, warnings)]
+#![doc(html_root_url = "https://docs.rs/tokio-executor/0.1.4")]
+
 //! Task execution related traits and utilities.
 //!
 //! In the Tokio execution model, futures are lazy. When a future is created, no
@@ -32,9 +35,6 @@
 //! [`Park`]: park/index.html
 //! [`Future::poll`]: https://docs.rs/futures/0.1/futures/future/trait.Future.html#tymethod.poll
 
-#![deny(missing_docs, missing_debug_implementations, warnings)]
-#![doc(html_root_url = "https://docs.rs/tokio-executor/0.1.3")]
-
 extern crate futures;
 
 mod enter;