Take advantage of 2018 edition.

- Remove explicit 'extern crate' fields where useful
- Fix mutability of variable (unrelated?)
- Add rbxmx dependency, which needs 2018 edition

server/src/bin.rs

@@ -1,8 +1,5 @@
 #[macro_use] extern crate clap;
 #[macro_use] extern crate log;
-extern crate env_logger;
-
-extern crate librojo;
 
 use std::{
     path::{Path, PathBuf},