Upgrade num-traits to 0.2.15

This project was upgraded with external_updater.
Usage: tools/external_updater/updater.sh update rust/crates/num-traits
For more info, check https://cs.android.com/android/platform/superproject/+/master:tools/external_updater/README.md

Test: TreeHugger
Change-Id: Ib712393e1371ed920552a4194b6aae0c0acd22d3

1 files changed, 0 insertions, 6 deletions

diff --git a/README.md b/README.md
index 1ee1e1e..3633962 100644
--- a/README.md
+++ b/README.md
@@ -16,12 +16,6 @@ Add this to your `Cargo.toml`:
 num-traits = "0.2"
 ```
 
-and this to your crate root:
-
-```rust
-extern crate num_traits;
-```
-
 ## Features
 
 This crate can be used without the standard library (`#![no_std]`) by disabling