Bump untrusted version for security

[tirkarthi]

Hi,

This project seems to use untrusted = 0.5 as I can see from the cargo.toml file on master branch. Recently a security issue was fixed as part of 0.6.2 release. Please refer to rustsec/advisory-db@3c0458d .

Kindly ignore if this is irrelevant or fixed on another branch.

Thanks.

[Keats]

Bumping it requires updating ring to an alpha version afaik though :/
Since only one version of ring is currently allowed in a project, it would be a breaking change to a version that lots of users might not be able to upgrade to and will need another breaking change to get rid of the alpha later.

@briansmith would you recommend updating to the alpha version of ring now or is the 0.13 release soonish?