Use Rust 1.38.0

[kseo]

Rust 1.38.0 is available. https://blog.rust-lang.org/2019/09/26/Rust-1.38.0.html

[remagpie]

Now rust 1.39.0 is available. https://blog.rust-lang.org/2019/11/07/Rust-1.39.0.html
How about using 1.39.0 instead?

[sgkim126]

#1850 made CodeChain use 1.39.0