Bump the required Rust version. Because of some transitive dependencies needing `std::sync::atomic::spin_loop_hint`, this won't compile without [1.24](https://blog.rust-lang.org/2018/02/15/Rust-1.24.html):
```
Compiling parking_lot_core v0.2.14
error[E0432]: unresolved import `std::sync::atomic::spin_loop_hint`
--> .cargo/registry/src/github.com-1ecc6299db9ec823/parking_lot_core-0.2.14/src/spinwait.rs:14:5
|
14 | use std::sync::atomic::spin_loop_hint;
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ no `spin_loop_hint` in `sync::atomic`
error: aborting due to previous error
error: Could not compile `parking_lot_core`.
warning: build failed, waiting for other jobs to finish...
error: failed to compile `bat v0.2.0`, intermediate artifacts can be found at `/var/folders/z6/9wkk458s4y1g3_82242l_wv5ws7k_8/T/cargo-install.Nyyc2EJDojxa`
```