Update Development dependencies example code.

To use the rust 1.30+ syntax for using crates, which is used elsewhere in the book.

Author: kraktus

src/testing/dev_dependencies.md

@@ -11,24 +11,20 @@ File `Cargo.toml`:
 ```toml
 # standard crate data is left out
 [dev-dependencies]
-pretty_assertions = "0.4.0"
+pretty_assertions = "1"
 ```
 
 File `src/lib.rs`:
 
 ```rust,ignore
-// externing crate for test-only use
-#[cfg(test)]
-#[macro_use]
-extern crate pretty_assertions;
-
 pub fn add(a: i32, b: i32) -> i32 {
     a + b
 }
 
 #[cfg(test)]
 mod tests {
     use super::*;
+    use pretty_assertions::assert_eq; // crate for test-only use. Cannot be used in non-test code.
 
     #[test]
     fn test_add() {