Update to 2018 edition. (#642)

This is the minimum fix required. It updates to use crate-relative
imports.

cocoa-foundation/Cargo.toml

@@ -7,6 +7,7 @@ repository = "https://github.com/servo/core-foundation-rs"
 version = "0.1.2"
 authors = ["The Servo Project Developers"]
 license = "MIT OR Apache-2.0"
+edition = "2018"
 
 [package.metadata.docs.rs]
 default-target = "x86_64-apple-darwin"

cocoa-foundation/src/foundation.rs

@@ -9,7 +9,7 @@
 
 #![allow(non_upper_case_globals)]
 
-use base::{id, nil, BOOL, NO, SEL};
+use crate::base::{id, nil, BOOL, NO, SEL};
 use block::Block;
 use libc;
 use std::os::raw::c_void;
@@ -32,7 +32,7 @@ const UTF8_ENCODING: usize = 4;
 
 #[cfg(target_os = "macos")]
 mod macos {
-    use base::id;
+    use crate::base::id;
     use core_graphics_types::base::CGFloat;
     use core_graphics_types::geometry::CGRect;
     use objc;