* async-await: fix README example dependencies
As per commit "async-await: track nightly changes (#661)" ( commit
2f690d30bc)
> The `tokio-async-await` crate is no longer a facade. Instead, the
> `tokio` crate provides a feature flag to enable async/await support.
Ensure the example in the async-await README file also works by
correctly declaring this updated dependency
* async-await: remove unnecessary 'edition' declaration from README
As the "edition" feature was stabilized in rust v1.30 and async-await
specifies that the nightly toolchain must be used, remove the use of the
"edition" feature gate since it is enabled by default.
