[beta]: Update Cargo's submodule

Hopefully a routine update to the rust-1.31.0 branch!

Author: alexcrichton

src/Cargo.lock

@@ -209,6 +209,7 @@ dependencies = [
  "crossbeam-utils 0.5.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "crypto-hash 0.3.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "curl 0.4.18 (registry+https://github.com/rust-lang/crates.io-index)",
+ "curl-sys 0.4.13 (registry+https://github.com/rust-lang/crates.io-index)",
  "env_logger 0.5.12 (registry+https://github.com/rust-lang/crates.io-index)",
  "failure 0.1.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "filetime 0.2.1 (registry+https://github.com/rust-lang/crates.io-index)",