Use of deprecated method

[marek22k]

When compiling, a warning appears that obsolete functions would be used.

warning: use of deprecated method `chrono::TimeZone::timestamp`: use `timestamp_opt()` instead
   --> src/bin/roughenough-client.rs:463:26
    |
463 |             let ts = Utc.timestamp(seconds as i64, nsecs as u32);
    |                          ^^^^^^^^^
    |
    = note: `#[warn(deprecated)]` on by default

warning: use of deprecated method `chrono::TimeZone::timestamp`: use `timestamp_opt()` instead
   --> src/bin/roughenough-client.rs:466:28
    |
466 |             let ts = Local.timestamp(seconds as i64, nsecs as u32);
    |                            ^^^^^^^^^

warning: `roughenough` (bin "roughenough-client") generated 2 warnings