Verify documentation links on Travis

Let's use cargo-deadlinks with the following command on Travis:

cargo deadlinks --check-http

This currently fails with some errors which are due to re-exports from std:

Found invalid urls in /home/stjepang/work/async-std/target/doc/async_std/io/type.Result.html:
        Linked file at path /home/stjepang/work/async-std/target/doc/async_std/result/enum.Result.html does not exist!
Found invalid urls in /home/stjepang/work/async-std/target/doc/async_std/io/struct.Error.html:
        Linked file at path /home/stjepang/work/async-std/target/doc/std/io/struct.Error.html does not exist!
        Linked file at path /home/stjepang/work/async-std/target/doc/std/io/enum.ErrorKind.html does not exist!
        Linked file at path /home/stjepang/work/async-std/target/doc/async_std/ffi/struct.NulError.html does not exist!

The way we resolve these errors is by writing shim docs for re-exports from std, similarly to how we did that here:

async-std/src/os/unix/net/mod.rs

Line 14 in addda39

if #[cfg(feature = "docs")] {

The idea is that under the docs feature flag we generate "fake" docs linking to async-std's types, but otherwise re-export real types from std.