State minimum version of rustc required to build

I develop some applications which must build within an offline schroot to be packaged on Pop / Ubuntu 18.04, which means I'm limited to using crates in our projects that can compile with a version of rustc no greater than 1.24.1. Seems this crate requires Rust 2018 edition, which is not backwards compatible, so I'll have to continue using failure_derive for now.