Backport rustls version upgrade as tokio-rustls 0.15.0

[Paolo Barbolini]

The lettre crate supports both tokio 0.2 and tokio 0.3. We'd like to support 1.0 too when it comes out, but we can't do so unless the tokio-rustls corresponding to each tokio version we support takes the same rustls version.

Could #37 be backported and released as 0.15.0, making it possible to get rustls 0.19 in tokio 0.2 projects?

[quininer]

Okay, I put it in the tokio-rustls/0.15 branch.

[Paolo Barbolini]

Thanks!