Fix compile error on latest Rust nightly

[techwizrd]

Due to an issue with ring v0.11.0 (used by cookie in rocket v0.3.5) and an update to the Rust compiler rustc, sks-rs v0.1.0 no longer builds successfully.

See the following:

• https://github.com/SergioBenitez/Rocket/issues/513
• https://github.com/briansmith/ring/issues/598
• https://github.com/briansmith/ring/issues/609

This should hopefully be fixed in a few days when the new release of Rust is out (2018-Jan-04).