Update the ring dev dependency version.

[gendx]

This updates the ring dependency to the latest available version. This also allows to check if the cargo audit warning is resolved by that.

• Tests pass
• Appropriate changes to README are included in PR

[gendx]

Note that the advisory is still in Travis logs (see also briansmith/ring#921).

Crate:  spin
Title:  spin is no longer actively maintained
Date:   2019-11-21
URL:    https://rustsec.org/advisories/RUSTSEC-2019-0031
Dependency tree: 
spin 0.5.2
├── ring 0.16.11
│   └── crypto 0.1.0
│       └── ctap2 0.1.0
└── linked_list_allocator 0.6.6
    └── libtock 0.1.0
        ├── ctap2 0.1.0
        └── crypto 0.1.0