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-timer/src/lib.rs

@@ -1,3 +1,6 @@
+#![doc(html_root_url = "https://docs.rs/tokio-timer/0.2.6")]
+#![deny(missing_docs, warnings, missing_debug_implementations)]
+
 //! Utilities for tracking time.
 //!
 //! This crate provides a number of utilities for working with periods of time:
@@ -22,9 +25,6 @@
 //! [`Interval`]: struct.Interval.html
 //! [`Timer`]: timer/struct.Timer.html
 
-#![doc(html_root_url = "https://docs.rs/tokio-timer/0.2.5")]
-#![deny(missing_docs, warnings, missing_debug_implementations)]
-
 extern crate tokio_executor;
 
 extern crate crossbeam_utils;