chore(dependencies): Force tokio to be at least 0.2.5 (#2186)
chore(dependencies): Force tokio to be at least 0.2.5 (#2186)

Without this, you can end up with tokio 0.2.4 and hyper 0.13.5 in your
project, leading to a compile error like this:

```
error[E0599]: no method named `try_recv` found for struct `tokio::sync::mpsc::unbounded::UnboundedReceiver<client::dispatch::Envelope<T, U>>` in the current scope
   --> src/client/dispatch.rs:161:26
    |
161 |         match self.inner.try_recv() {
    |                          ^^^^^^^^ method not found in `tokio::sync::mpsc::unbounded::UnboundedReceiver<client::dispatch::Envelope<T, U>>`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0599`.
error: could not compile `hyper`.
```
hyperium/hyper,hyperium/hyper,tshepang/hyper,tshepang/hyper,hyperium/hyper,tshepang/hyper