Upgrade to Rust 1.31.0 (#2052)

* Upgrade to Rust 1.31.0
* Upgrade nightly
* Fix all clippy warnings
* Revert relaxed version check and update

vote-signer/src/bin/main.rs

@@ -23,7 +23,8 @@ fn main() -> Result<(), Box<error::Error>> {
                 .value_name("NUM")
                 .takes_value(true)
                 .help("JSON RPC listener port"),
-        ).get_matches();
+        )
+        .get_matches();
 
     let port = if let Some(p) = matches.value_of("port") {
         p.to_string()

vote-signer/src/rpc.rs

@@ -46,10 +46,9 @@ impl VoteSignerRpcService {
                     sleep(Duration::from_millis(100));
                 }
                 server.unwrap().close();
-                ()
             })
             .unwrap();
-        VoteSignerRpcService { thread_hdl, exit }
+        Self { thread_hdl, exit }
     }
 
     pub fn exit(&self) {