Push msrv to 1.40 and update approx to 0.5 and uom to 0.31

[ghost]

Since rust 1.38 a new cargo.lock format has been decide and they convert every cargo.lock to it. So it is impossible to compile with stable and going back below 1.38. rust-lang/cargo#7579

Rust 1.40(16-12-2019) permit to have approx and uom update.