Minor Clippy fix

[alex-ter]

As I didn't get any reply in the discussion, and I see Clippy fixes in the git log, I decided to simply go ahead and submit this one.

Fixes this;

warning: unnecessary use of `get("library_crate").is_none()`
   --> cargo-auditable/tests/it.rs:178:18
    |
178 |     assert!(bins.get("library_crate").is_none());
    |             -----^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |             |
    |             help: replace it with: `!bins.contains_key("library_crate")`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_get_then_check

BTW, the test_wasm test fails on master. I haven't looked any deeper into why, but also don't see any open issues for that. Is the failure expected? Would it be helpful if I looked into that?

[Shnatsel]

The test probably fails because you don't have the webassembly target installed on your system.