Pass --format-version 1 to cargo metadata.

Suppress warning introduced by rust-lang/cargo#3841.

src/bootstrap/metadata.rs

@@ -58,6 +58,7 @@ fn build_krate(build: &mut Build, krate: &str) {
     // the dependency graph and what `-p` arguments there are.
     let mut cargo = Command::new(&build.cargo);
     cargo.arg("metadata")
+         .arg("--format-version").arg("1")
          .arg("--manifest-path").arg(build.src.join(krate).join("Cargo.toml"));
     let output = output(&mut cargo);
     let output: Output = json::decode(&output).unwrap();