Update `spin` crate dependency version

[mrcosta]

We're using cargo audit in our project and today this vulnerability was found:

error: Vulnerable crates found!

ID:	 RUSTSEC-2019-0013
Crate:	 spin
Version: 0.4.8
Date:	 2019-08-27
URL:	 https://github.com/mvdnes/spin-rs/issues/65
Title:	 Wrong memory orderings in RwLock potentially violates mutual exclusion
Solution: upgrade to: >= 0.5.2

I will fork the project and try to see if just updating spin don't break anything and submit a PR if doesn't.