`cargo fix` still warns about `rust_2018_preview`

This flag is not supposed to be used for the beta release, but:

```console
> > cargo fix --edition
    Checking foo v0.1.0 (C:\Users\steve\tmp\foo)
warning: failed to find `#![feature(rust_2018_preview)]` in `src\lib.rs`
this may cause `cargo fix` to not be able to fix all
issues in preparation for the 2018 edition
```