#[allow(clippy::assertions_on_constants)] does not work on older versions of Rust

[soc]

This change: 75e817f

Broke the build of the [directories](https://github.com/soc/directories-rs) (and dirs) crates:
https://travis-ci.org/soc/directories-rs/jobs/592813734

Could this code be replaced by a normal import of clippy, such that it keeps working on existing Rust versions?

[Alex Crichton]

Thanks for the report but this is a change in a test and this crate doesn't work to support 4-year-old verisons of Rust right now.