update async-tungstenite to enable ppc builds

[adamthiede]

I'm attempting to package magic-wormhole.rs for Alpine Linux, and it's failing the powerpc build because of a dependency chain. async-tungstenite depends on async-tls, which depends on rustls, which depends on ring. ring prior to 0.17 did not support ppc: briansmith/ring#1904

Updating to the latest async-tungstenite removes the dependency on the older version of ring and should allow this to build on ppc.

[felinira]

Thanks, looking good :)

Test failures seem to be due to too old MSRV, will bump it after merge.