Vulnerability in memoffset.

[afck]

cargo audit warns:

error: Vulnerable crates found!

ID:	 RUSTSEC-2019-0011
Crate:	 memoffset
Version: 0.2.1
Date:	 2019-07-16
URL:	 https://github.com/Gilnaa/memoffset/issues/9#issuecomment-505461490
Title:	 Flaw in offset_of and span_of causes SIGILL, drops uninitialized memory of arbitrary type on panic in client code
Solution: upgrade to: >= 0.5.0

error: 1 vulnerability found!

However, this is an indirect dependency:

$ cargo tree -ip memoffset
memoffset v0.2.1
└── crossbeam-epoch v0.7.1
    ├── crossbeam v0.7.1
    │   [dev-dependencies]
    │   └── hbbft v0.1.1 (/home/andreas/git/hbbft)
    │       └── hbbft_testing v0.1.0 (/home/andreas/git/hbbft/hbbft_testing)
    │           [dev-dependencies]
    │           └── hbbft v0.1.1 (/home/andreas/git/hbbft) (*)
    ├── crossbeam-deque v0.6.3
    │   ├── rayon v1.1.0
    │   │   └── reed-solomon-erasure v3.1.1
    │   │       └── hbbft v0.1.1 (/home/andreas/git/hbbft) (*)
    │   └── rayon-core v1.5.0
    │       └── rayon v1.1.0 (*)
    └── crossbeam-deque v0.7.1
        └── crossbeam v0.7.1 (*)

Let's disable cargo audit in ci.sh for now, until we can fix this.

Hopefully we won't need to do anything, if crossbeam releases a minor version with the fix: crossbeam-rs/crossbeam#402