Include Utc in imports

err.bash

cbzehner at MacBook-Pro-4 ~/Projects/pull-requests/rust/exercises/gigasecond                                                                 [SUCCESS]
$cargo check
   Compiling gigasecond v1.1.0 (file:///Users/cbzehner/Projects/pull-requests/rust/exercises/gigasecond)
error[E0412]: cannot find type `Utc` in this scope
 --> src/lib.rs:6:30
  |
6 | pub fn after(start: DateTime<Utc>) -> DateTime<Utc> {
  |                              ^^^ not found in this scope
help: possible candidates are found in other modules, you can import them into scope
  |
3 | use chrono::Utc;
  |
3 | use chrono::offset::Utc;
  |
3 | use chrono::prelude::Utc;
  |

error[E0412]: cannot find type `Utc` in this scope
 --> src/lib.rs:6:48
  |
6 | pub fn after(start: DateTime<Utc>) -> DateTime<Utc> {
  |                                                ^^^ not found in this scope
help: possible candidates are found in other modules, you can import them into scope
  |
3 | use chrono::Utc;
  |
3 | use chrono::offset::Utc;
  |
3 | use chrono::prelude::Utc;
  |

warning: unused import: `Duration`
 --> src/lib.rs:3:24
  |
3 | use chrono::{DateTime, Duration};
  |                        ^^^^^^^^
  |
  = note: #[warn(unused_imports)] on by default

error: aborting due to 2 previous errors

error: Could not compile `gigasecond`.

To learn more, run the command again with --verbose.