Consider removing version_check

[matklad]

This crate currently uses version_check to support Rust versions older than 1.36.0. This is ancient history by Rust's standards, so perhaps it is worth it to remove build.rs and version_check altogether. The motivation being compilation-time performance --- one less deb and one less build.rs would shave off some milliseconds. Not a huge win, of course, but still a meaningful reduction, given crate's popularity.