Seems difficult to get a minimal set of features working

I am trying to reduce the dependencies of our project and I tried to use the minimal set of features for it.

I get this error:

```
error[E0425]: cannot find function `block_on` in module `task`
--> src/gstreamer.rs:18:15
|
18 | task::block_on(async move {
| ^^^^^^^^ not found in `task`

error[E0425]: cannot find function `sleep` in module `task`
--> src/gstreamer.rs:79:11
|
79 | task::sleep(std::time::Duration::from_secs(1)).await;
| ^^^^^ not found in `task`
|
help: possible candidate is found in another module, you can import it into scope
|
6 | use std::thread::sleep;
|

error[E0425]: cannot find function `spawn` in module `task`
--> src/gstreamer.rs:93:11
|
93 | task::spawn(feed_pipeline(playbin.clone(), manifest));
| ^^^^^ not found in `task`
|
help: possible candidate is found in another module, you can import it into scope
|
6 | use std::thread::spawn;
|

error[E0425]: cannot find function `block_on` in module `async_std::task`
--> src/main.rs:52:1
|
52 | #[async_std::main]
| ^^^^^^^^^^^^^^^^^^ not found in `async_std::task`
|
= note: this error originates in an attribute macro (in Nightly builds, run with -Z macro-backtrace for more info)
```

and I am setting it as:

```
# Cargo.toml
async-std = { version = "1.5.0", default-features = false, features = ["std", "attributes", "unstable"] }
```