compile with rust nightly

[emberian]

rust nightly errors out that the lint is already part of the rust_2018_idioms:

error[E0453]: warn(unused_extern_crates) incompatible with previous forbid in same scope
  --> /Users/ember/.cargo/registry/src/github.com-1ecc6299db9ec823/atomic_enum-0.1.1/src/lib.rs:13:5
   |
2  |     rust_2018_idioms,
   |     ---------------- `forbid` level set here
...
13 |     unused_extern_crates,
   |     ^^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error

[brandenburg]

Thank you for fixing this; I need it as well. In case anyone wonders, here's some context to the change.