Fix regression by updating Cargo.toml to depend on mio-serial 5.0.2

[astraw]

This fixes ff8d9ec, which introduces a
dependency on mio-serial 5.0.2 but did not bump the version requirement
in Cargo.toml. Without also updating Cargo.toml to depend on mio-serial
5.0.2, upgrading a project with an old lock file to latest tokio-serial will break with

error[E0432]: unresolved imports `mio_serial::SerialPortType`, `mio_serial::UsbPortInfo`
  --> /builds/straw/proj/.cargo-proj/registry/src/github.com-1ecc6299db9ec823/tokio-serial-5.4.2/src/lib.rs:13:40
   |
13 |     SerialPortBuilder, SerialPortInfo, SerialPortType, StopBits, UsbPortInfo,
   |                                        ^^^^^^^^^^^^^^            ^^^^^^^^^^^ no `UsbPortInfo` in the root
   |                                        |
   |                                        no `SerialPortType` in the root
   |                                        help: a similar name exists in the module: `SerialPort`

[estokes]

Oops

[astraw]

Thanks for the quick fix and new release!