Dependency `futures` is out-of-date

The `examples/two.rs` will fail if using latest `futures`(version = 0.3.30). It will pass using version show in `Cargo.toml` (version = 0.1.15).
The error log(already remove some line from `examples/two.rs`):
```
Compiling playground v0.1.0 (/home/dead_bush_sanpai/workspace/code/playground)
warning: unused import: `futures::Future`
--> src/main.rs:11:5
|
11 | use futures::Future;
| ^^^^^^^^^^^^^^^
|
= note: `#[warn(unused_imports)]` on by default

error[E0599]: no method named `join` found for struct `Perform` in the current scope
--> src/main.rs:33:39
|
33 | let requests = session.perform(a).join(session.perform(b));
| ^^^^ method not found in `Perform`
|
::: /home/dead_bush_sanpai/.cargo/registry/src/mirrors.ustc.edu.cn-61ef6e0cd06fb9b8/futures-0.1.31/src/future/mod.rs:716:8
|
716 | fn join(self, other: B) -> Join
| ---- the method is available for `Perform` here
|
= help: items from traits can only be used if the trait is in scope
help: the following trait is implemented but not in scope; perhaps add a `use` for it:
|
6 + use futures::future::Future;
|

For more information about this error, try `rustc --explain E0599`.
warning: `playground` (bin "playground") generated 1 warning
error: could not compile `playground` (bin "playground") due to previous error; 1 warning emitted
```