unnecessary "nightly" feature

[leeopop]

Recent change says that "async-await" no longer requires nightly.
However, building alpha.18 version claims that "nightly" feature is not on.
I'm not sure whether removing this feature gate is correct or not, thus simply reporting the compilation error I have.

error: The `async-await` feature requires the `nightly` feature as an explicit opt-in to unstable features
  --> /home/leeopop/.cargo/registry/src/github.com-1ecc6299db9ec823/futures-util-preview-0.3.0-alpha.18/src/lib.rs:21:1
   |
21 | compile_error!("The `async-await` feature requires the `nightly` feature as an explicit opt-in to unstable features");
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

[Nemo157]

This is referring to the nightly cargo feature for the futures crate that was used to gate unstable features (since non-std crates have nothing really like the unstable attribute). That has since been renamed to unstable to make it more obvious and is no longer required when using the async-await feature. Those changes will be in the next alpha release.

[leeopop]

Oh, I was working on alpha.18 while looking master's README.
I'm looking forward to see the next release!