Outdated futures version

Project fails to build with the current `futures` version.

```
Compiling futures v0.3.4
error[E0432]: unresolved import `futures_core::core_reexport`
--> /Users/nskobelevs/.cargo/registry/src/index.crates.io-6f17d22bba15001f/futures-0.3.4/src/lib.rs:542:9
|
542 | pub use futures_core::core_reexport;
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^ no `core_reexport` in the root

error[E0433]: failed to resolve: could not find `document_join_macro` in `futures_util`
--> /Users/nskobelevs/.cargo/registry/src/index.crates.io-6f17d22bba15001f/futures-0.3.4/src/lib.rs:561:15
|
561 | futures_util::document_join_macro! {
| ^^^^^^^^^^^^^^^^^^^ could not find `document_join_macro` in `futures_util`

error[E0433]: failed to resolve: could not find `document_select_macro` in `futures_util`
--> /Users/nskobelevs/.cargo/registry/src/index.crates.io-6f17d22bba15001f/futures-0.3.4/src/lib.rs:584:15
|
584 | futures_util::document_select_macro! {
| ^^^^^^^^^^^^^^^^^^^^^ could not find `document_select_macro` in `futures_util`

error[E0603]: module `async_await` is private
--> /Users/nskobelevs/.cargo/registry/src/index.crates.io-6f17d22bba15001f/futures-0.3.4/src/lib.rs:547:23
|
547 | pub use futures_util::async_await;
| ^^^^^^^^^^^ private module
|
note: the module `async_await` is defined here
--> /Users/nskobelevs/.cargo/registry/src/index.crates.io-6f17d22bba15001f/futures-util-0.3.21/src/lib.rs:34:1
|
34 | mod async_await;
| ^^^^^^^^^^^^^^^
```

This version has since been yanked - the next non yanked version is 0.3.11 which seems to work fine.