Update to Rust 2018 edition

src/main.rs

@@ -1,9 +1,3 @@
-extern crate chrono;
-extern crate clap;
-extern crate libudev;
-extern crate serde;
-extern crate serde_json;
-
 use chrono::Duration;
 use clap::App;
 use clap::Arg;

src/mountinfo.rs

@@ -3,7 +3,7 @@ use std::io;
 use std::io::prelude::*;
 use std::path;
 
-use error::Error;
+use crate::error::Error;
 
 pub struct MountInfo {
     pub mount_id: u32,