Upgrade spin crate to patch a vulnerability.

The vulnerability:
```
$ ./build-support/bin/ci.py --cargo-audit
...
error: Vulnerable crates found!

ID:	 RUSTSEC-2019-0013
Crate:	 spin
Version: 0.5.1
Date:	 2019-08-27
URL:	 mvdnes/spin-rs#65
Title:	 Wrong memory orderings in RwLock potentially violates mutual exclusion
Solution: upgrade to: >= 0.5.2

error: 1 vulnerability found!
Cargo audit failure
```

Although we don't directly depend on `spin`, we depend on `lazy_static`
(amongst others) which does:
```
$ (cd src/rust/engine && ../../../build-support/bin/native/cargo tree -p spin -i)
spin v0.5.2
├── lazy_static v1.3.0
...
```

So this change was generated with a targeted upgrade:
```
$ ./build-support/bin/native/cargo update --manifest-path src/rust/engine/Cargo.toml -p spin --aggressive
```