Fix #32

[jgraef]

Fixes #32

1. Added futures-03-sink to async-tungstenite's features.
2. Fixed a bug in async-tungstenite (see PR) and changed the dependency to use that PR for now

I'll wait with merging this to see if we can get rid of the git dependency.

[jgraef]

async-tungstenite merged the patch and I bumped the dependency.

But when compiling the wasm example I get:

error[E0310]: the parameter type `Fut` may not live long enough
  --> /home/emma/.cargo/registry/src/index.crates.io-6f17d22bba15001f/futures-util-0.3.30/src/stream/futures_unordered/task.rs:81:9
   |
81 |         waker_ref(this)
   |         ^^^^^^^^^^^^^^^
   |         |
   |         the parameter type `Fut` must be valid for the static lifetime...
   |         ...so that the type `Fut` will meet its required lifetime bounds
   |
help: consider adding an explicit lifetime bound
   |
78 | impl<Fut: 'static> Task<Fut> {
   |         +++++++++

I got this error before while investigating the async-tungstenite bug, but could not reproduce.

[jgraef]

This error is caused by some dependency pulling in futures-util 0.3.30, which has been yanked (might be related to this). Pinning futures-util to >=0.3.31 fixes this.