Update build dependencies and edition

Use the 2021 Rust edition. To use current version of clap changes were
adapted.

src/posix/recv.rs

@@ -1,13 +1,13 @@
 use anyhow::Result;
 use chrono::{DateTime, Local};
-use clap::Clap;
+use clap::Parser;
 use humantime::Duration;
 use log::info;
 use posixmq::PosixMq;
 use std::str;
 
 /// Receive and print a message from a message queue
-#[derive(Clap, Debug)]
+#[derive(Debug, Parser)]
 pub struct Recv {
     /// Do not block
     #[clap(short, long)]