add forgotten use std::ascii::AsciiExt;

[symphorien]

If I compile master, I get:

error[E0599]: no method named `eq_ignore_ascii_case` found for type `&str` in the current scope
  --> /home/symphorien/.cargo/registry/src/github.com-1ecc6299db9ec823/which-1.0.4/src/lib.rs:37:24
   |
37 |             .map(|e| e.eq_ignore_ascii_case(env::consts::EXE_EXTENSION)) {
   |                        ^^^^^^^^^^^^^^^^^^^^
   |
   = help: items from traits can only be used if the trait is in scope
   = note: the following trait is implemented but not in scope, perhaps add a `use` for it:
           candidate #1: `use std::ascii::AsciiExt;`

error: aborting due to previous error

error: Could not compile `which`.

Using the tip of the compiler, here is a fix :)
Tests still pass.
Please release a new version on crates.io after merging :)

[harryfei]

Sorry for that and thanks for this PR. I have merged it and pulblished a new version in crates.io (which @ 1.0.5).
I sudden remove this line because the compile warning it as unused. After I dig into this bug, I found the root cause.
The use std::ascii::AsciiExt line is not needed on the latest Rust versoin. However when we remove it, the older version of Rust will build failed. Finally, I merge this pull request and add #[allow(unused)] to bypass it.

FWIW, more detail in rust-lang/rust#46510.

[symphorien]

thanks a lot !