Update to Rust 1.38

Author: carols10cents

.travis.yml

@@ -3,7 +3,7 @@ dist: trusty
 language: rust
 cache: cargo
 rust:
-  - 1.37.0
+  - 1.38.0
 branches:
   only:
   - master

listings/ch02-guessing-game-tutorial/listing-02-02/Cargo.lock

@@ -6,7 +6,7 @@ warning: unused `std::result::Result` that must be used
 10 |     io::stdin().read_line(&mut guess);
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
-   = note: #[warn(unused_must_use)] on by default
+   = note: `#[warn(unused_must_use)]` on by default
    = note: this `Result` may be an `Err` variant, which should be handled
 
     Finished dev [unoptimized + debuginfo] target(s) in 0.59s

listings/ch02-guessing-game-tutorial/no-listing-02-without-expect/output.txt

@@ -6,7 +6,7 @@ error[E0308]: mismatched types
 7 | fn plus_one(x: i32) -> i32 {
   |    --------            ^^^ expected i32, found ()
   |    |
-  |    this function's body doesn't return
+  |    implicitly returns `()` as its body has no tail or `return` expression
 8 |     x + 1;
   |          - help: consider removing this semicolon
   |